11 research outputs found
Proton Conductivity Control by Ion Substitution in a Highly Proton-Conductive Metal–Organic Framework
Proton conductivity through two-dimensional
(2-D) hydrogen-bonding
networks within a layered metal–organic framework (MOF) (NH<sub>4</sub>)<sub>2</sub>(H<sub>2</sub>adp)[Zn<sub>2</sub>(ox)<sub>3</sub>]·3H<sub>2</sub>O (H<sub>2</sub>adp = adipic acid; ox = oxalate)
has been successfully controlled by cation substitution. We synthesized
a cation-substituted MOF, K<sub>2</sub>(H<sub>2</sub>adp)[Zn<sub>2</sub>(ox)<sub>3</sub>]·3H<sub>2</sub>O, where the ammonium ions in
a well-defined hydrogen-bonding network are substituted with non-hydrogen-bonding
potassium ions, without any apparent change in the crystal structure.
We successfully controlled the proton conductivity by cleavage of
the hydrogen bonds in a proton-conducting pathway, showing that the
2-D hydrogen-bonding networks in the MOF truly contribute to the high
proton conductivity. This is the first example of the control of proton
conductivity by ion substitution in a well-defined hydrogen-bonding
network within a MOF
Proton-Conductive Magnetic Metal–Organic Frameworks, {NR<sub>3</sub>(CH<sub>2</sub>COOH)}[M<sub>a</sub><sup>II</sup>M<sub>b</sub><sup>III</sup>(ox)<sub>3</sub>]: Effect of Carboxyl Residue upon Proton Conduction
Proton-conductive magnetic metal–organic frameworks (MOFs),
{NR<sub>3</sub>(CH<sub>2</sub>COOH)}[M<sub>a</sub><sup>II</sup>M<sub>b</sub><sup>III</sup>(ox)<sub>3</sub>] (abbreviated as <b>R–M</b><sub><b>a</b></sub><b>M</b><sub><b>b</b></sub>:
R = ethyl (Et), <i>n</i>-butyl (Bu); M<sub>a</sub>M<sub>b</sub> = MnCr, FeCr, FeFe) have been studied. The following six
MOFs were prepared: <b>Et–MnCr</b>·2H<sub>2</sub>O, <b>Et–FeCr</b>·2H<sub>2</sub>O, <b>Et–FeFe</b>·2H<sub>2</sub>O, <b>Bu–MnCr</b>, <b>Bu–FeCr</b>, and <b>Bu–FeFe</b>. The structure of <b>Bu–MnCr</b> was determined by X-ray crystallography. Crystal data: trigonal, <i>R</i>3<i>c</i> (#161), <i>a</i> = 9.3928(13)
Å, <i>c</i> = 51.0080(13) Å, <i>Z</i> = 6. The crystal consists of oxalate-bridged bimetallic layers interleaved
by {NBu<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> ions. <b>Et–MnCr</b>·2H<sub>2</sub>O and <b>Bu–MnCr</b> (R–MnCr
MOFs) show a ferromagnetic ordering with <i>T</i><sub>C</sub> of 5.5–5.9 K, and <b>Et–FeCr</b>·2H<sub>2</sub>O and <b>Bu–FeCr</b> (R–FeCr MOFs) also
show a ferromagnetic ordering with <i>T</i><sub>C</sub> of
11.0–11.5 K. <b>Et–FeFe</b>·2H<sub>2</sub>O and <b>Bu–FeFe</b> (R–FeFe MOFs) belong to
the class II of mixed-valence compounds and show the magnetism characteristic
of Néel N-type ferrimagnets. The Et-MOFs (<b>Et–MnCr</b>·2H<sub>2</sub>O, <b>Et–FeCr</b>·2H<sub>2</sub>O and <b>Et–FeFe</b>·2H<sub>2</sub>O) show high
proton conduction, whereas the Bu–MOFs (<b>Bu–MnCr</b>, <b>Bu–FeCr</b>, and <b>Bu–FeFe</b>) show
moderate proton conduction. Together with water adsorption isotherm
studies, the significance of the carboxyl residues as proton carriers
is revealed. The R–MnCr MOFs and the R–FeCr MOFs are
rare examples of coexistent ferromagnetism and proton conduction,
and the R–FeFe MOFs are the first examples of coexistent Néel
N-type ferrimagnetism and proton conduction
Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks
We controlled the hydrophilicity of metal–organic
frameworks
(MOFs) to achieve high proton conductivity and high adsorption of
water under low humidity conditions, by employing novel class of MOFs,
{NR<sub>3</sub>(CH<sub>2</sub>COOH)}[MCr(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated as <b>R-MCr</b>, where
R = Me (methyl), Et (ethyl), or Bu (<i>n</i>-butyl), and
M = Mn or Fe): <b>Me-FeCr</b>, <b>Et-MnCr</b>, <b>Bu-MnCr</b>, and <b>Bu-FeCr</b>. The cationic components have a carboxyl
group that functions as the proton carrier. The hydrophilicity of
the cationic ions was tuned by the NR<sub>3</sub> residue to decrease
with increasing bulkiness of the residue: {NMe<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NEt<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NBu<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup>. The
proton conduction of the MOFs increased with increasing hydrophilicity
of the cationic ions. The most hydrophilic sample, <b>Me-FeCr</b>, adsorbed a large number of water molecules and showed a high proton
conductivity of ∼10<sup>–4</sup> S cm<sup>–1</sup>, even at a low humidity of 65% relative humidity (RH), at ambient
temperature. Notably, this is the highest conductivity among the previously
reported proton-conducting MOFs that operate under low RH conditions
Promotion of Low-Humidity Proton Conduction by Controlling Hydrophilicity in Layered Metal–Organic Frameworks
We controlled the hydrophilicity of metal–organic
frameworks
(MOFs) to achieve high proton conductivity and high adsorption of
water under low humidity conditions, by employing novel class of MOFs,
{NR<sub>3</sub>(CH<sub>2</sub>COOH)}[MCr(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated as <b>R-MCr</b>, where
R = Me (methyl), Et (ethyl), or Bu (<i>n</i>-butyl), and
M = Mn or Fe): <b>Me-FeCr</b>, <b>Et-MnCr</b>, <b>Bu-MnCr</b>, and <b>Bu-FeCr</b>. The cationic components have a carboxyl
group that functions as the proton carrier. The hydrophilicity of
the cationic ions was tuned by the NR<sub>3</sub> residue to decrease
with increasing bulkiness of the residue: {NMe<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NEt<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup> > {NBu<sub>3</sub>(CH<sub>2</sub>COOH)}<sup>+</sup>. The
proton conduction of the MOFs increased with increasing hydrophilicity
of the cationic ions. The most hydrophilic sample, <b>Me-FeCr</b>, adsorbed a large number of water molecules and showed a high proton
conductivity of ∼10<sup>–4</sup> S cm<sup>–1</sup>, even at a low humidity of 65% relative humidity (RH), at ambient
temperature. Notably, this is the highest conductivity among the previously
reported proton-conducting MOFs that operate under low RH conditions
Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework
Structure-defined
metal–organic frameworks (MOFs) are of
interest because rational design and construction allow us to develop
good proton conductors or possibly control the proton conductivity
in solids. We prepared a highly proton-conductive MOF (NH<sub>4</sub>)<sub>2</sub>(adp)[Zn<sub>2</sub>(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b><sub><b>2</b></sub><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having
definite crystal structures and showing reversible structural transformations
among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b><sub><b>2</b></sub><b>O</b>), and trihydrate (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) phases. The crystal structures
of all of these phases were determined by X-ray crystallography. Hydrogen-bonding
networks consisting of ammonium ions, water molecules, and carboxylic
acid groups of the adipic acids were formed inside the two-dimensional
interlayer space in hydrated <b>1·2H</b><sub><b>2</b></sub><b>O</b> and <b>1·3H</b><sub><b>2</b></sub><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>2<sub>1</sub>/<i>c</i>, No. 14) was changed into
that of <b>1·3H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>1̅, No. 2), depending on water content
because of rearrangement of guests and acidic molecules. Water molecules
play a key role in proton conduction as conducting media and serve
as triggers to change the proton conductivity through reforming hydrogen-bonding
networks by water adsorption/desorption processes. Proton conductivity
was consecutively controlled in the range from ∼10<sup>–12</sup> S cm<sup>–1</sup> (<b>1</b>) to ∼10<sup>–2</sup> S cm<sup>–1</sup> (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) by the humidity. The relationships among the
structures of conducting pathways, adsorption behavior, and proton
conductivity were investigated. To the best of our knowledge, this
is the first example of the control of a crystalline proton-conducting
pathway by guest adsorption/desorption to control proton conductivity
using MOFs
Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework
Structure-defined
metal–organic frameworks (MOFs) are of
interest because rational design and construction allow us to develop
good proton conductors or possibly control the proton conductivity
in solids. We prepared a highly proton-conductive MOF (NH<sub>4</sub>)<sub>2</sub>(adp)[Zn<sub>2</sub>(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b><sub><b>2</b></sub><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having
definite crystal structures and showing reversible structural transformations
among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b><sub><b>2</b></sub><b>O</b>), and trihydrate (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) phases. The crystal structures
of all of these phases were determined by X-ray crystallography. Hydrogen-bonding
networks consisting of ammonium ions, water molecules, and carboxylic
acid groups of the adipic acids were formed inside the two-dimensional
interlayer space in hydrated <b>1·2H</b><sub><b>2</b></sub><b>O</b> and <b>1·3H</b><sub><b>2</b></sub><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>2<sub>1</sub>/<i>c</i>, No. 14) was changed into
that of <b>1·3H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>1̅, No. 2), depending on water content
because of rearrangement of guests and acidic molecules. Water molecules
play a key role in proton conduction as conducting media and serve
as triggers to change the proton conductivity through reforming hydrogen-bonding
networks by water adsorption/desorption processes. Proton conductivity
was consecutively controlled in the range from ∼10<sup>–12</sup> S cm<sup>–1</sup> (<b>1</b>) to ∼10<sup>–2</sup> S cm<sup>–1</sup> (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) by the humidity. The relationships among the
structures of conducting pathways, adsorption behavior, and proton
conductivity were investigated. To the best of our knowledge, this
is the first example of the control of a crystalline proton-conducting
pathway by guest adsorption/desorption to control proton conductivity
using MOFs
Control of Crystalline Proton-Conducting Pathways by Water-Induced Transformations of Hydrogen-Bonding Networks in a Metal–Organic Framework
Structure-defined
metal–organic frameworks (MOFs) are of
interest because rational design and construction allow us to develop
good proton conductors or possibly control the proton conductivity
in solids. We prepared a highly proton-conductive MOF (NH<sub>4</sub>)<sub>2</sub>(adp)[Zn<sub>2</sub>(ox)<sub>3</sub>]·<i>n</i>H<sub>2</sub>O (abbreviated to <b>1·</b><i><b>n</b></i><b>H</b><sub><b>2</b></sub><b>O</b>, adp: adipic acid, ox: oxalate, <i>n</i> = 0, 2, 3) having
definite crystal structures and showing reversible structural transformations
among the anhydrate (<b>1</b>), dihydrate (<b>1·2H</b><sub><b>2</b></sub><b>O</b>), and trihydrate (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) phases. The crystal structures
of all of these phases were determined by X-ray crystallography. Hydrogen-bonding
networks consisting of ammonium ions, water molecules, and carboxylic
acid groups of the adipic acids were formed inside the two-dimensional
interlayer space in hydrated <b>1·2H</b><sub><b>2</b></sub><b>O</b> and <b>1·3H</b><sub><b>2</b></sub><b>O</b>. The crystal system of <b>1</b> or <b>1·2H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>2<sub>1</sub>/<i>c</i>, No. 14) was changed into
that of <b>1·3H</b><sub><b>2</b></sub><b>O</b> (<i>P</i>1̅, No. 2), depending on water content
because of rearrangement of guests and acidic molecules. Water molecules
play a key role in proton conduction as conducting media and serve
as triggers to change the proton conductivity through reforming hydrogen-bonding
networks by water adsorption/desorption processes. Proton conductivity
was consecutively controlled in the range from ∼10<sup>–12</sup> S cm<sup>–1</sup> (<b>1</b>) to ∼10<sup>–2</sup> S cm<sup>–1</sup> (<b>1·3H</b><sub><b>2</b></sub><b>O</b>) by the humidity. The relationships among the
structures of conducting pathways, adsorption behavior, and proton
conductivity were investigated. To the best of our knowledge, this
is the first example of the control of a crystalline proton-conducting
pathway by guest adsorption/desorption to control proton conductivity
using MOFs
Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion
We
demonstrate a metal–organic framework (MOF) design for
the inclusion of hydroxide ions. Salt inclusion method was applied
to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework)
to introduce alkylammonium hydroxides as ionic carriers. We found
that tetrabutylammonium salts are immobilized inside the pores by
a hydrophobic interaction between the alkyl groups of the salt and
the framework, which significantly increases the hydrophilicity of
ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity
for OH<sup>–</sup> ion exchange, implying that freely exchangeable
OH<sup>–</sup> ions are present in the MOF. ZIF-8 containing
OH<sup>–</sup> ions showed an ionic conductivity of 2.3 ×
10<sup>–8</sup> S cm<sup>–1</sup> at 25 °C, which
is 4 orders of magnitude higher than that of the blank ZIF-8. This
is the first example of an MOF-based hydroxide ion conductor
Design and Synthesis of Hydroxide Ion–Conductive Metal–Organic Frameworks Based on Salt Inclusion
We
demonstrate a metal–organic framework (MOF) design for
the inclusion of hydroxide ions. Salt inclusion method was applied
to an alkaline-stable ZIF-8 (ZIF = zeolitic imidazolate framework)
to introduce alkylammonium hydroxides as ionic carriers. We found
that tetrabutylammonium salts are immobilized inside the pores by
a hydrophobic interaction between the alkyl groups of the salt and
the framework, which significantly increases the hydrophilicity of
ZIF-8. Furthermore, ZIF-8 including the salt exhibited a capacity
for OH<sup>–</sup> ion exchange, implying that freely exchangeable
OH<sup>–</sup> ions are present in the MOF. ZIF-8 containing
OH<sup>–</sup> ions showed an ionic conductivity of 2.3 ×
10<sup>–8</sup> S cm<sup>–1</sup> at 25 °C, which
is 4 orders of magnitude higher than that of the blank ZIF-8. This
is the first example of an MOF-based hydroxide ion conductor
Proton Conduction Study on Water Confined in Channel or Layer Networks of La<sup>III</sup>M<sup>III</sup>(ox)<sub>3</sub>·10H<sub>2</sub>O (M = Cr, Co, Ru, La)
Proton
conduction of the La<sup>III</sup>M<sup>III</sup> compounds, LaM(ox)<sub>3</sub>·10H<sub>2</sub>O (abbreviated to <b>LaM</b>; M
= Cr, Co, Ru, La; ox<sup>2–</sup> = oxalate) is studied in
view of their networks. <b>LaCr</b> and <b>LaCo</b> have
a ladder structure, and the ladders are woven to form a channel network. <b>LaRu</b> and <b>LaLa</b> have a honeycomb sheet structure,
and the sheets are combined to form a layer network. The occurrence
of these structures is explained by the rigidness versus flexibility
of [M(ox)<sub>3</sub>]<sup>3–</sup> in the framework with large
La<sup>III</sup>. The channel networks of <b>LaCr</b> and <b>LaCo</b> show a remarkably high proton conductivity, in the range
from 1 × 10<sup>–6</sup> to 1 × 10<sup>–5</sup> S cm<sup>–1</sup> over 40–95% relative humidity (RH)
at 298 K, whereas the layer networks of <b>LaCr</b> and <b>LaCo</b> show a lower proton conductivity, ∼3 × 10<sup>–8</sup> S cm<sup>–1</sup> (40–95% RH, 298 K).
Activation energy measurements demonstrate that the channels filled
with water molecules serve as efficient pathways for proton transport. <b>LaCo</b> was gradually converted to La<sup>III</sup>Co<sup>II</sup>(ox)<sub>2.5</sub>·4H<sub>2</sub>O, which had no channel structure
and exhibited a low proton conductivity of less than 1 × 10<sup>–10</sup> S cm<sup>–1</sup>. The conduction–network
correlation of LaCo(ox)<sub>2.5</sub>·4H<sub>2</sub>O is reported