Electrochemically Mediated Syntheses of Titanium(III)-Based Metal-Organic Frameworks

Article abstract of DOI:10.1021/jacs.9b05035

Although metal-organic frameworks featuring coordinatively unsaturated transition metal sites are relatively common, examples with redox-active cations are rare. In this report, we describe the electrochemically mediated synthesis of Ti^III-MIL-101 from the inexpensive Ti⁴⁺ precursor TiCl₄. The framework obtained via electrosynthesis is identical to that prepared from the significantly more expensive and air-sensitive starting material TiCl₃. The above electrosynthetic strategy was also extended to prepare Ti^III-MIL-100 and two high-quality extended Ti^III-MIL structures, for the first time. These materials represent examples of titanium-based MOFs with extended pore structures. Several physical methods demonstrate that these materials are superior in quality to samples of the analogous MOFs prepared via conventional routes from starting exogenous TiCl₃. Given the ease with which the electrosyntheses may be carried out and their compatibility with a broad range of bridging ligands, we expect that this new methodology will find utility for the synthesis of a number of novel materials containing coordinatively unsaturated, redox-active metal cations.

Full text of DOI:10.1021/jacs.9b05035

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Electrochemically Mediated Syntheses of Titanium(III)-Based Metal−
Organic Frameworks
Alexandra M. Antonio, Joel Rosenthal,* and Eric D. Bloch*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: Although metal−organic frameworks fea-
turing coordinatively unsaturated transition metal sites are
relatively common, examples with redox-active cations are
rare. In this report, we describe the electrochemically
mediated synthesis of TiIII-MIL-101 from the inexpensive
Ti4+ precursor TiCl4. The framework obtained via
electrosynthesis is identical to that prepared from the
significantly more expensive and air-sensitive starting
material TiCl3. The above electrosynthetic strategy was
also extended to prepare TiIII-MIL-100 and two high-
quality extended TiIII-MIL structures, for the first time.
These materials represent examples of titanium-based
MOFs with extended pore structures. Several physical
methods demonstrate that these materials are superior in
quality to samples of the analogous MOFs prepared via
conventional routes from starting exogenous TiCl3. Given
the ease with which the electrosyntheses may be carried
out and their compatibility with a broad range of bridging
ligands, we expect that this new methodology will find
utility for the synthesis of a number of novel materials
containing coordinatively unsaturated, redox-active metal
cations.
Figure 1. Portions of the MIL-100 and MIL-101 structure types that
etal−organic frameworks (MOFs) have garnered
depict the large pores in the materials. Use of a linear dicarboxylate
such as H2bdc generates the MIL-101 structure; use of a triangular
tricarboxylate such as H3btc generates the MIL-100 structure.
Mconsiderable attention as they display high surface
areas,1−3 tunable pore chemistry,4−6 and can be prepared from
virtually any transition metal cation. These factors distinguish
MOFs as highly tunable for numerous applications.7 Of the
The preparation of MOFs that are redox active and/or based
upon metals in nontraditional or difficult to access oxidation
states generally relies on the use of air-sensitive starting
materials, necessitating the use of rigorously air-free reaction
conditions and limiting materials discovery throughput.
Moreover, for many metal ions in reduced oxidation states,
the number of available starting materials is limited, as the
synthesis of a metal−organic framework often relies on
screening a number of parameters, including counteranion
identity, placing additional bottlenecks on discovery of new
structures.23 In addressing these limitations, electrochemical
methods present an intriguing alternative strategy for synthesis
of reduced MOF materials.
broad material space that is occupied by MOFs, those featuring
accessible metal cations on their pore surface are particularly
interesting as such systems can facilitate selective gas binding,8
high-density gas storage,9 sensing,10 and serve as sites for
small-molecule activation and catalysis.11 However, particularly
for the latter which require redox-active metal cation sites, the
number of reported materials is limited.12−16 For example,
although numerous Cr3+ MOFs exist, there are few examples
containing this metal in the 2+ oxidation state.17 Similarly,
many examples of Ti4+ metal−organic frameworks have been
reported,18−21 but to date there is a single Ti3+-based MOF.22
This material, TiIII-MIL-101 (Figure 1), is based on trimeric
clusters connected to terephthalic acid (H2bdc) and features a
high BET surface area, exposed Ti3+ sites, and strong Ti−O2
interactions via the formation of peroxide adducts. Further, its
exposed Ti3+ sites are substrate accessible and can strongly
bind gas molecules such as CO2 and H2.
Redox reactivity has been coupled to synthesis of MOFs. For
example, redox chemistry has been used to drive ligand
deprotonation reactions coupled to MOF syntheses.24 Addi-
Received: May 10, 2019
© XXXX American Chemical Society
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and 0.1 M TBAPF6 as supporting electrolyte in a 10:1 DMF/
EtOH mixture using a nickel foam working electrode, followed
by heating at 120 °C for 18 h afforded a crystalline dark purple
powder in high yield.
Powder X-ray diffraction experiments indicated the resulting
material is isostructural to the previously reported TiIII-MIL-
101 phase (Figure S6). The structure is comprised of trimeric
Ti3+ clusters connected to form tetrahedral building units with
bdc2− anions on their vertices. The tetrahedral units are edge
linked to afford a structure containing two large mesoporous
cages, one with both pentagonal and hexagonal windows and
the other comprised solely of pentagonal windows. TiIII-MIL-
101 obtained via the previously reported solvothermal method
from starting Ti3+ displays a BET (Langmuir) surface area of
2970 (4440) m2/g.22 The corresponding material synthesized
via electrochemical generation of Ti3+ from TiCl4 has a BET
(Langmuir) surface area of 3285 (4360) m2/g. Although
PXRD, SEM, XPS, and TGA analyses indicate the
solvothermally and electrochemically generated materials are
virtually indistinguishable, synthesis of the latter is accom-
plished using inexpensive TiCl4, as opposed to relying on
exogenously prepared and highly air-sensitive TiCl3. Of further
note is the fact that commercial TiCl4 is routinely available in
99.9% (and higher) purity, while TiCl3 is typically prepared via
aluminum reduction that result in Ti3+ starting materials
containing appreciable levels of Al3+.
Figure 2. (a) Cyclic voltammogram recorded of TiCl4 (1.0 mM) in
10:1 DMF:EtOH containing 0.1 M TBAPF6 at a scan rate of 100
mV/s. (b) UV−vis absorption spectra of a TiCl4/H2bdc DMF:EtOH
solution containing 0.1 M TBAPF6 during CPE at Eapplied = −1.20 V.
(c) Change in absorbance at λ = 650 nm during CPE described in
(b). (d) Total current density and overall Faradaic efficiency for TiCl4
and H+ reduction from TiCl4/H2bdc DMF:EtOH solution (Eapplied
−1.20 V).
=
A particular advantage of electrochemically synthesized
reduced metal frameworks is the management of proton
balance throughout the reaction. Significant concentrations of
H+ are generated during the course of solvothermal MOF
syntheses as ligands are deprotonated to form M−L bonds. As
many MOF syntheses are dependent on pH, the continually
increasing proton concentration can complicate synthesis of
phase-pure materials. The electrosynthetic strategy described
above provides a convenient means to manage H+ build up
associated with formation of the bdc2− anions needed for TiIII-
MIL-101 synthesis. The nickel foam working electrodes used
to generate the Ti3+ from TiCl4 show excellent HER activity at
significantly lower potentials than those used for the reduction
of Ti4+.29 Accordingly, CPE of the TiCl4/H2bdc solutions with
the nickel cathode not only generates Ti3+, but also reduces the
carboxylic acid protons to generate the bdc2− ligands and H2,
which is evolved from the cathode compartment.
To follow the production of H2 over the course of the CPE,
we monitored the reaction headspace during TiIII-MIL-101
electrosynthesis via gas chromatography.30,31 Initial formation
of H2 was detected within as little as 5 min and plateaued after
∼3−5 h. Initial Faradaic efficiencies for HER were <20% and
slowly increased to ∼70% as concentrations of Ti4+ were
depleted to generate Ti3+. In accounting for both the reduction
of TiCl4 to Ti3+ and H2bdc (or H3btc, vide infra) protons to
H2, we consistently observed net current efficiencies in excess
of 75% over the course of the entire CPE.
tionally, electrochemical dissolution of metallic anodes to
generate oxidized metal ions (i.e., M0 → Mn+), has been
utilized to synthesize Cu2+ and Zn2+ based MOFs.25−28
Despite these examples, we are aware of no published
examples in which electrochemistry has been leveraged to
alter the oxidization state of a dissolved metal ion in solution
prior to MOF synthesis. Such a strategy is attractive as it can
allow for the oxidation state of dissolved ions to be rapidly
tuned via controlled potential electrolysis (CPE) for the
targeted synthesis of MOFs containing metal centers in
nontraditional, reduced oxidation states. Additionally, this
strategy permits the large number of high oxidation state metal
starting materials that are inexpensive and readily accessible to
be converted to higher value reduced forms and facilitate the
discovery of new materials containing metal ions in reactive
oxidation states. This method also has the advantage that a
larger number of reaction conditions can be screened, as they
can be set up in the presence of air and then deoxygenated
prior to electrochemical reduction. To this end, we targeted
the synthesis of novel Ti3+ frameworks given their scarcity and
potential for small molecule storage and activation.
The Ti3+ based MOF TiIII-MIL-101 can be prepared by
reaction of exogenous TiCl3 with terephthalic acid (H2bdc) at
120 °C in 10:1 N,N-dimethylformamide (DMF):ethanol for
18 h under rigorously air-free conditions. In order to prepare
this material from Ti3+ generated electrochemically from Ti4+
sources, we utilized cyclic voltammetry (CV) to determine
potentials (Figures S1−S3) at which Ti3+ can be generated
from a series of Ti4+ based sources (e.g., TiCl4, Ti(OiPr)4,
Cp2TiCl2). In particular, CVs recorded for solutions of TiCl4
(Figure 2a) showed a cathodic wave corresponding to Ti4+
reduction at Ecat = −1.20 V (all potentials reported versus Ag/
AgNO3). Preparative scale reduction of TiCl4 by CPE at Eapplied
= −1.20 V using a nickel electrode could be monitored by
UV−vis spectroscopy (Figure 2b,c). The complete reduction
of starting Ti4+ to Ti3+ was accomplished in as little as 60 min.
A 4 h CPE (Eapplied = −1.20 V) of TiCl4 in the presence H2bdc
Given the potentially broad applicability of MOF electro-
synthesis from soluble metal precursors, we sought to employ
this method to expand the library of Ti3+ MOFs by targeting
other multitopic carboxylate-based ligands. Since the μ3-O
centered trimeric cluster is ubiquitous to a large number of
MIL-type MOFs, we targeted H3btc-based MIL-100. This
structure has previously been reported for trivalent Al,32 Sc,33
V,34 Cr,35 Fe,36 and mixed-metal systems and features
connectivity that is analogous to the MIL-101 structure type,
but where ligands occupy sites on the faces rather than vertices
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Figure 3. (a) N2 adsorption in TiIII-MIL-100 at 77 K of
electrochemically (blue) and solvothermally synthesized (black)
materials. (b) PXRD patterns of electrochemically (blue) and
solvothermally synthesized (red) TiIII-MIL-100 as compared to the
predicted powder pattern (black). SEM images for (c) electrochemi-
cally and (d) solvothermally synthesized TiIII-MIL-100.
of the tetrahedral building units. The electrochemical
reduction of a TiCl4/H3btc DMF:EtOH (10:1) solution
containing 0.1 M TBAPF6 followed by reaction at 120 °C
for 18 h afforded the target material, as confirmed by powder
X-ray diffraction (Figure 3). Interestingly, identical solvother-
mal reaction conditions utilizing commercial TiCl3 rather than
electrochemically reduced TiCl4 afforded MOF powders with
significantly lower uniformity (Figure 3). After thorough
solvent exchanges and activation at 100 °C the solvothermally
and electrochemically prepared materials showed BET
(Langmuir) surface areas of 1304 (1568) and 1736 (2708)
m2/g, respectively (Figure 3a). The latter value is in good
agreement with those reported for analogous MIL-100
materials.33,35
Despite several attempts, solvothermal syntheses utilizing
exogenous TiCl3 as starting material consistently afforded
frameworks with lower surface areas compared to those
obtained using Ti3+ that had been electrochemically generated
from TiCl4. The lower surface areas obtained for the
solvothermal materials may coincide with a Ti4+-based
framework with MIL-100 topology exhibiting a BET surface
area of 1321 m2/g that was recently reported.37 We note that
XPS analysis of solvothermally prepared samples of both TiIII-
MIL-101 and TiIII-MIL-100 show that each retain chloride as a
charge-balancing anion in the frameworks. Electrochemically
prepared samples of these two Ti3+-based frameworks both
contain fluoride anions (in place of chloride), which is
presumed to originate from the TBAPF6 supporting electro-
lyte.
Figure 4. SEM images for (a) electrochemically and (b)
solvothermally synthesized TiIII-MIL-101-bpdc. (c) N2 adsorption in
TiIII-MIL-101-bpdc at 77 K. SEM images for (d) electrochemically
and (e) solvothermally synthesized TiIII-MIL-100-tatb. (f) N2
adsorption in TiIII-MIL-100-tatb at 77 K. (Blue data = electrochemi-
cally prepared samples from TiCl4; black data = solvothermally
prepared samples from exogenous TiCl3).
Given the near complete dearth of Ti3+-based metal−organic
frameworks, we sought to further expand the scope of this
method to extended pore structures. Although materials of this
type based on the M3O building block have been reported,
they typically feature surface areas that are significantly lower
than values that are calculated for pristine structures. MIL-
101(Fe)_BPDC, which is a biphenyldicarboxylate-based
material that adopts the MIL-101 structure type, displays a
BET surface area of just 210 m2/g as compared to the
theoretical value of 4500 m2/g.38 A MIL-100 structure based
on an expanded tritopic (H3btb) linker displays a BET surface
area of just 26 m2/g versus the expected value of 3990 m2/g.38
By adopting the straightforward electrosynthetic strategy
developed for TiIII-MIL-101 and TiIII-MIL-100 (vide supra),
we have been able to isolate high-quality extended pore Ti3+
based MOFs for the first time. Electrochemical reduction of a
TiCl4 solution containing either 4,4′-biphenyldicarboxylic acid
(H2bpdc) or 4,4′,4′′-[1,3,5]triazine-2,4,6-triyl-tris-benzoic acid
(H3tatb) followed by heating at 120 °C for 18 h afforded TiIII-
MIL-101-bpdc and TiIII-MIL-100-tatb, respectively. TiIII-MIL-
101-bpdc displays a BET surface area of 3263 m2/g and a pore
volume of 2.20 cm3/g. As shown in Table S2, these values are
∼50% higher than those realized for samples of TiIII-MIL-101-
bpdc prepared solvothermally from exogenous TiCl3 and
H2bpdc (2139 m2/g and 1.32 cm3/g). Similarly, electrochemi-
cally synthesized TiIII-MIL-100-tatb displays a BET surface
area and pore volume of 5842 m2/g and 4.04 cm3/g (Figure
4). These values dwarf those realized for analogous
solvothermally prepared samples (740 m2/g and 0.49 cm3/
g). The pore volume of the electrosynthesized TiIII-MIL-100-
To determine whether the presence of supporting electrolyte
improved the surface area of the electrochemically prepared
TiIII-MIL-100, TBAPF6 was added to the solvothermal reaction
mixture (exogenous TiCl3/H3btc). The thusly obtained dark
brown product was both amorphous and nonporous. Further,
solvothermal syntheses with exogenous TiCl3/H3btc consis-
tently afforded materials of smaller particle size (Figure 3d) as
compared to syntheses utilizing electrochemically reduced
TiCl4 (Figure 3c).
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tatb (note: this structure is also referred to as PCN-333)21 is in
excellent agreement with the value reported for the analogous
Al-based MOF, PCN-333 (3.81 cm3/g).39 The electrochemi-
cally synthesized materials also feature larger and more well-
defined particle sizes (Figure 4) and increased crystallinities
(Figures S10, S11).
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In conclusion, we have leveraged electrochemical methods
to synthesize reduced metal−organic frameworks. Electro-
chemical synthesis of TiIII-MIL-101, TiIII-MIL-100, TiIII-MIL-
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b05035.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: joelr@udel.edu.
*E-mail: edb@udel.edu.
ORCID
Joel Rosenthal: 0000-0002-6814-6503
Eric D. Bloch: 0000-0003-4507-6247
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Benjamin Trump for help with powder X-ray
diffraction experiments. This work was supported by a
University of Delaware Research Foundation-Strategic Ini-
tiatives Grant to E.D.B. and J.R. E.D.B. also thanks the
University of Delaware for startup funds that made this work
possible. This research used resources of the Advanced Photon
Source, a U.S. Department of Energy (DOE) Office of Science
User Facility operated for the DOE Office of Science by
Argonne National Laboratory under Contract No. DE-AC02-
06CH11357. We thank the staff of 17-BM for help with
synchrotron X-ray data collection. A portion of this work was
supported by the National Institutes of Health under award
number P20GM104316. Faradaic efficiencies were determined
using instrumentation obtained through NSF CHE1352120.
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DOI: 10.1021/jacs.9b05035
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