Expandable porous organic frameworks with built-in amino and hydroxyl functions for CO2 and CH4 capture

Article abstract of DOI:10.1039/c8cc03951h

The synthesis of porous organic 3D frameworks, wherein amine, hydroxyl and Li-alkoxide functions were built directly on the monomer-unit carbon core, realizes improved interactions with target gases. CO₂ was retained by the amine group with a r

Full text of DOI:10.1039/c8cc03951h

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PEREGO, D. PIGA, S. Bracco and P. Sozzani, Chem. Commun., 2018, DOI: 10.1039/C8CC03951H.
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Expandable Porous Organic Frameworks with Built-In Amino and
Hydroxyl Functions for CO2 and CH4 Capture
J. Perego, D. Piga, S. Bracco, P. Sozzani and A. Comotti*
Abstract. The synthesis of Porous Organic 3D Frameworks, permanent motif of the frameworks is constituted by three p-
wherein amine, hydroxyl and Li-alkoxide functions were built phenylene rings protruding from a central core, a forth
directly on the monomer-unit carbon core, realizes improved substituent being the chosen organic function directly bonded
interactions with target gases. CO2 was retained by the amine to the same core, yielding low-density architectures
group with a remarkable energy of 54 kJ mol-1, while 2D MAS NMR denominated Triphenylmethane Aromatic Frameworks (TAFs).
provided rare evidence of amine-to-gas short-distance By this synthetic strategy, the branches sustain the porous
interactions. Frameworks containing hydroxyl and Li-alkoxide network and the functions protrude towards the empty spaces
functions show optimal interaction energy with CH4 up to 25 kJ of the pores (Scheme 1). The functions are regularly bound
mol-1. The light network of 3-branch building units ensures
expandability of the nano-sponges.
Gas capture and storage is a prominent issue for our society,
since it enables solutions for environmental and energy
problems, especially for the target gases carbon dioxide and
methane, both massively present in the atmosphere and in the
lithosphere, and their connection to power and heat
onto the aliphatic carbon and do not modify the reactivity of
generation.1 Porous materials can efficiently store
a
the aromatic groups during the condensation reaction.
Scheme 1. Chemical structure of TAFs (above). Schematic representation of the porous
organic polymer with walls decorated with –NH2 groups (below).
considerable amount of such gases with a moderate energy
consumption for the gas release, thus ensuring a sound energy
balance in sorption/release cycles.2
The organic functions are exposed to the gases diffused into
the framework. As a result, the behavior of the 3D frameworks
with hydroxyl and aliphatic amine groups (TAF-OH and TAF-
NH2) is quite distinct from that of the fully hydrocarburic
network (TAF): the amine functionalized compound exhibits an
outstanding affinity for CO2, while Li-alkoxide-containing
framework (TAF-OLi) shows an enhanced affinity for CH4. Solid
state NMR provided unique evidence for close proximity of the
gas to the walls, and the CO2 interaction with the nanopore
environment. The present work is part of the effort being
spent for obtaining a gas affinity modulated by the wall nature
and POF functionalization.
In recent years there has been an increasing number of porous
materials obtained by exploiting the formation of a variety of
interactions and chemical bonds, which span from soft
interactions up to metal-organic and covalent bonds.3 Among
these families, porous organic frameworks (POFs) boast some
prerogatives derived by the stability of their 3D network of
covalent bonds. These prerogatives include the absence of
potentially toxic metal ions, high thermal robustness,
resistence to solvents, especially water, and remarkable
volumetric and gravimetric pore capacity.4 Further important
properties are enabled by the post-synthetic insertion of
organic functional-groups, each promoting specific interactions
with target gases.4 Thus, in the search for enhanced
interactions by innovative solutions, we prepared new porous
organic frameworks, starting from simple 3D synthons bearing
organic functions. In our design, tetrahedral cores bear both
the branches and the functional groups, which will be retained
in the skeleton of the resulting framework. Therefore, the
Monomers to be polymerized by Ni-catalysed Yamamoto
coupling reaction require bromine atoms substituted on the
aromatic rings in para-position. The presence of three such
sacrificial functions on each monomer ensures a large number
of cross-links, according to the Flory’s polycondensation law,
which dictates particle gelation will occur as soon as functional
group conversion (p) overcomes the value of 0.66 (DPn(inf.)
=
2/(2-3p)).5 However, reaction proceeds beyond this point,
leading to less than 2-3% unreacted bromine substituents
(ESI). Powder XRD revealed no short-range periodicity, as
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typically occurs in covalent frameworks obtained by and 3.2 mmol g-1 at 273 K in TAF-NH2: these values appear
irreversible bond formation (ESI). Solid state MAS NMR spectra attractive for applications, also considering the relatively low
of the powders recorded on 13C and 1H nuclei indicate the cost of triphenylmethane and its derivatives and the high
formation of carbon-carbon bonds between the aromatic rings thermal stability of the frameworks.
and highlight the dramatic change of tetrahedral carbon
chemical shift due to the presence of hydrogen or organic-
function substitution. The 13C quantitative spectra evaluation
confirm the full retention of -OH and -NH2 substituents (1 per
monomer unit) upon polymerization, as expected by
Yamamoto coupling (Figure 1). This is an indication for optimal
regularity in terms of constant stoichiometry and controlled
insertion of functional groups on the overall framework.
Infrared spectroscopy confirms the presence of the respective
functional groups attached to the quaternary carbon atoms at
the core of the monomer units, e.g. the band at 1158 cm-1
highlights the C-O stretching of the tertiary alcohol while the
bands at 3320 and 3380 cm-1 indicate the presence of the
primary amine group (ESI). The high stability of the
frameworks (over 400°C) was demonstrated by TGA analysis.
N2 adsorption isotherms, collected at 77
K respectively
provided Langmuir and BET surface areas of 1565, 1343, 1108
m2 g-1 and 1383, 1190, 984 m2 g-1 for TAF, TAF-NH2 and TAF-OH
(Figure 1). The surface areas and pore-volumes (0.95, 0.68 and
0.45 cm3 g-1 for TAF, TAF-NH2 and TAF-OH), despite the space
occupied by the organic functions in the pores,. are
outstanding and comparable (or superior) to many of the best
performing functionalized POFs.6 Indeed, there was
a
considerable contribution of mesoporosity to the total pore
volume. N2 desorption branches run distinctly above the
adsorption curves, especially for TAF, forming large hysteresis
loops indicative of the framework swellability with an
increased pore capacity of more than 40% after expansion.
Structure expandability of TAF in the presence of well-
interacting ‘solvents’ was visually observed in the expansion in
volume from dry to soaked powder (Figure 1f,g). After filtering
the imbibed sample, the increase in weight was evaluated
quantitatively to be 370% for acetone, 470% for hexane and
740% for ethylacetate. Unlike the frameworks produced by
Friedel-Craft reaction, in which flexibility was introduced by
linear connecting chains,4e,7 we achieved flexible frameworks
through the intrinsic connectivity among the monomer units.
Indeed, the number of branches protruding from the
monomer units is limited to 3 in TAFs, as opposed to 4 in the
rigid PAFs,4a allowing for a larger conformational adaptability.
Particle size distribution, as determined by Dynamic Light
Scattering in dilute suspensions, follow Gaussian profiles
centered at 40 nm, with 96% of the particles restricted to the
Figure 1. a) N2 adsorption/desorption isotherms at 77 K of TAF (blue circles), TAF-OH
(red triangles) and TAF-NH2 (green diamonds). 13C MAS NMR spectra of b) TAF, c) TAF-
NH2 and d) TAF-OH. e) Particle size distribution of TAF, TAF-NH2 and TAF-OH (blue,
green and red lines, respectively). f,g) Images of dry and soaked TPAF powder in
ethylacetate, respectively.
In general, these are competitive with the best performing
porous materials of similar pore capacity, irrespective of the
family they belong to: covalent, metal-organic or molecular
crystals.2a,3a,6a The adsorption isotherms recorded at three
distinct temperatures and elaborated by van’t Hoff equation,
yielded the adsorption energy at low coverage as high as 54 kJ
mol-1 (13 kcal mol-1) for the amine derivative, which
outperforms the other two compounds (26 kJ mol-1 and 30 kJ
mol-1 for TAF and TAF-OH) and is comparable to the highest
values of amine-containing POPs.6a The strong interaction of
TAF-NH2 was accounted for by the favorable activity of the
aliphatic amines. Our choice to exploit aliphatic primary amine
as the active component of the framework was successful and
10-100 nm range, irrespective of the
consistently with the preservation of
3
compounds,
a
constant and
homogeneous reaction course even in the presence of
diversified functions in the monomer. This nanometric particle
size is considered of interest for cell internalization and is thus
suitable for drug-delivery.8
CO2 adsorption isotherms recorded at three distinct
temperatures (273K, 283K and 298K) revealed excellent
uptakes already at low pressure (Figure 2). The uptake up to 1
bar is relevant, reaching 1.9 mmol g-1 at ambient temperature
2 | J. Name., 2012, 00, 1-3
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confirms their outstanding effectiveness for CO2 capture and notable value matches, or even exceeds, the performance of
MOFs i.e. Ni-MOF-74 (21.4 kJ mol-1), PCN-14 (18.7 kJ mol-1) and
HKUST-1 (17 kJ mol-1).13a
sequestration.
Figure 3. 2D 1H-13C HETCOR MAS NMR spectrum of TAF-NH2 loaded with 13C-enriched
CO2 at 215K. A contact time of 5 ms was applied. 1D 13C CP MAS NMR spectrum and 1D
1H projection of
the sample are
reported.
Figure 2. a) CO2 isotherms of TAF, TAF-NH2 and TAF-OH (273K: blue, green and red
circles; 298 K: blue, green and red diamonds, respectively) and N2 adsorption isotherms
TAF, TAF-NH2 and TAF-OH (273 K: blue, green and red triangles; 298 K: blue, green and
red squares, respectively). b) Isosteric heat of adsorption of TAF-NH2, TAF-OH and TAF
(green, red and blue circles, respectively).
The direct spectroscopic observation of the intimate spatial
relationship between the CO2 gas and the NH2 groups exposed
to the cavity walls in the porous matrix was provided by 2D 1H-
13C heterocorrelated MAS NMR (Figure 3). The spectrum
recorded at 215 K shows through-space cross-correlations9
1
between H of the matrix and 13C-enriched CO2 carbons which
are depleted of hydrogens (δ(CO2)= 125.1 ppm). Thus, CO2 was
found in the slow exchange regime with the interacting sites
on the matrix walls, and spent a relatively long time over an
individual site, producing an efficient magnetization transfer
by dipole-dipole interactions that took place at distances as
short as 5-10 Å.10 In the 2D-heterocorrelated spectrum, NH2
hydrogens correlate with CO2 carbons, indicating that the CO2
molecules sit in close contact with the amine group.11 Also, the
NH2 interaction site is surrounded by aromatic hydrogens that
correlate with CO2. These results are rare spectroscopic
observations, supporting the highly energetic binding of CO2 in
porous materials.10,12
Figure 4. a) CH4 isosteric heat of adsorption of TAF-OLi, TAF-OH, TAF-NH2 and TAF
(orange, red, green and blue circles, respectively). b) CH4 adsorption isotherms at room
temperature and up to 100 bar of TAF, TAF-NH2 and TAF-OH (blue, green and red
circles, respectively), the value of the total adsorption volume over grams is reported
versus pressure. In the inset CH4 adsorption isotherm of TAF-OH: the value of the total
adsorption volume over volume is reported versus pressure. The black line represents
the differentce between the total adsorption value and the pure compressed CH4 while
the dashed line the pure compressed CH4.
The CH4 adsorption measurements collected up to 10 bar and
variable temperature (273, 283 and 298 K) indicate stimulating
sorption values (ESI). The binding energies at low coverage
exhibit values of 18 and 19 kJ mol-1 for TAF and TAF-NH2,
respectively, and up to 21 kJ for TAF-OH (Figure 4). The
intermolecular interactions were further increased by the
transformation of the -OH group into Li-alkoxide by treatment
with lithium hydride in THF. The compound, denonimated TAF-
OLi, showed excellent CH4 binding energy performance,
reaching a value as high as 25 kJ mol-1 (Figure 4). This quite
Interest in the delivery of large amounts of natural gas has led
to the exploration of middle/high pressure solutions in the
presence of absorbent materials, thus, methane isotherms of
TAFs have been recorded up to 100 bar and room temperature
(Figure 4). The volumetric uptake, that is important for
Adsorbed Natural Gas Technology (ANG), is shown in Figure 4.
The results are encouraging since a total adsorption value of
100 cm3cm-3 is achieved at 38 bar with a gain of 64 cm3 cm-3
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significantly only under extreme conditions. The adsorption
values at 35 and 65 bar are of 95 and 119 cm3cm-3, comparable
to POPs and exceeding the performance of COF-8, COF-513b
and DUT-13, MOF-210 and MOF-200.13c
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The investigation of porous frameworks generated by
branched monomers moved us to the design of 3D aromatic
frameworks (TAFs), starting from monomers with three rigid
aromatic branches and a functional group connected to the
same tetrahedral core. This arrangement ensures the
generation of a robust covalent scaffold, resistant to structural
collapse, and with well-anchored organic functions, regularly
spaced all over the framework. The newly-designed strategy
satisfies the requirements for creating a framework that
supports –OH, -NH2 and –O-Li+, which are constantly exposed
to the galleries. The frameworks were proved effective in
capturing CO2 and, especially, the amine derivative contains an
energetic binding site in each monomer unit accounting for 54
KJ mol-1. The direct spectroscopic observation of CO2 over the
sites was provided by the magnetization transfer from amine
hydrogens to CO2 carbon nuclei through a short-distance
dipolar interaction. Methane was a second target gas due to
its importance in energy supply, since its transportation can
cause concern on both large and small scales. Indeed, the
functional materials proposed here, besides the extraordinary
chemical and thermal stability, exhibit relevant sorptive
properties and excellent capacity, even in high-pressure
regimes, owing to the balance between micro- and meso-
porosity. From the applicative point of view, it is worthwhile
mentioning the relatively low cost of triphenylmethane and
derivatives, which constitute the building blocks of the TAF
frameworks. Further properties of the present frameworks
include network swellability for the creation of functional gels,
and the formation of copolymers containing complementary
organic groups, to access specialized functions.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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Organic functions, built on the node of a porous covalent architecture, exhibit excellent
affinity for CO2 (54 kJ mol-1) and CH4 (25 kJ mol-1): CO2 interacting favorably with amine
groups was observed by 2D NMR.