Porous organic polymers with anchored aldehydes: A new platform for post-synthetic amine functionalization en route for enhanced CO2 adsorption properties

Article abstract of DOI:10.1039/c3cc48228f

A novel porous organic polymer has been synthesized using the molecular building block approach to deliberately encompass aldehyde functionalities amenable to post functionalization. The resultant porous framework allows a facile, one-step quantitative and post-synthetic functionalization by amines, permitting enhanced CO₂ sorption properties.

Full text of DOI:10.1039/c3cc48228f

Volume 50 Number 16 25 February 2014 Pages 1923–2062
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Mohamed Eddaoudi et al.
Porous organic polymers with anchored aldehydes: a new platform
for post-synthetic amine functionalization en route for enhanced CO2
adsorption properties

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Porous organic polymers with anchored
aldehydes: a new platform for post-synthetic
amine functionalization en route for enhanced
CO2 adsorption properties†
Cite this: Chem. Commun., 2014,
50, 1937
Received 27th October 2013,
Accepted 26th November 2013
Vincent Guillerm, Łukasz J. Weselinski, Mohamed Alkordi, M. Infas H. Mohideen,
´
DOI: 10.1039/c3cc48228f
Youssef Belmabkhout, Amy J. Cairns and Mohamed Eddaoudi*
www.rsc.org/chemcomm
A novel porous organic polymer has been synthesized using the to use solid porous materials, such as zeolites,4 metal–organic frame-
molecular building block approach to deliberately encompass works (MOFs)5 or silica/carbon based adsorbents,6 as a support/host
aldehyde functionalities amenable to post functionalization. The for amine pending moieties. Indeed, targeting selective CO2 adsorp-
resultant porous framework allows a facile, one-step quantitative tion at relatively low pressures within microporous–mesoporous
and post-synthetic functionalization by amines, permitting enhanced solids strengthened by accessible and fine-tunable amine groups
CO2 sorption properties.
has proven to be a suitable alternative solution.7 This approach
permits efficient CO2 separation from dry as well as humid N2, O2,
Separation and purification of gases to produce commodities are CH4 and H2 containing streams with high selectivity and without the
regarded as energy intensive processes accounting for nearly 15% drawbacks associated with the solution-based amine approach.7a
of the global energy consumption in 2010 and projected to triple by
Porous organic polymers (POPs), an emerging class of materials
the year 2050.1 Admittedly, more efficient and cost-effective separa- constructed from the assembly of judiciously selected organic mole-
tion technologies are critical toward a balanced overall energy cular building blocks (MBBs), are gaining momentum as a promising
consumption map worldwide. Particularly, there is an urgent need class of gas adsorbent materials with notable potential to address
to efficiently separate and capture carbon dioxide (CO2) from flue many enduring societal challenges pertaining to energy and environ-
gas and valuable commodities, i.e. methane (CH4) and hydrogen mental sustainability.6,8 POPs are unique due to the potential and
(H2).1 Correspondingly, the last decades witnessed an exponential ability to mutually control their porous system structure (apparent
and alarming CO2 concentration rise in the atmosphere, to reach surface areas up to 6000 m2 gÀ1),9 composition and functionality.
unparalleled CO2 concentration levels in 2013 with a recorded Accordingly, and due to their usually high chemical and thermal
value in May 2013 of 400 parts per million (ppm), an unprece- stability, POPs are certainly placed as potential porous material
dented level in human history.2
candidates that can effectively address the carbon capture challenge.6
Accordingly, the necessity to reduce CO2 emissions from fossil Construction of a given POP platform can be accomplished via the
fuel-fired power plants as well as from the transport sector has directed assembly of judiciously selected MBBs, corresponding to the
triggered purposeful research programs in academia and industry vertex figures of a particularly targeted augmented net, and by
alike to economically and efficiently separate CO2 from nitrogen (N2), controlling the synthetic conditions to afford crystalline materials.10
the major component in flue gas. It is worth mentioning that the However, it has been shown that (i) POP crystallinity is not a
current CO2 capture technology is based on corrosive alkanolamine prerequisite to control their resultant porosity,11 and (ii) the ability
solutions, a process that remains costly and energy intensive, mainly to tune a given porous platform, introducing accessible functional
due to the high energy penalty associated with the regeneration of groups with strong affinity toward CO2 without jeopardizing/impair-
amines in aqueous solution.3 A promising alternative to reduce the ing the porosity, is key to achieve efficient CO2 capture.
energy expense, initially inspired by the aforementioned technology, is
Access to MOFs, as adsorbents for CO2 capture and separation
applications with enhanced CO2 sorption uptakes, energetics and
selectivity, has been accomplished through amine grafting on
unsaturated metal sites (UMS).12 Similarly, POPs emerged as a
suitable support for amine grafting using a two-step post-
synthetic modification (PSM) procedure, but requiring harsh
chemical treatments.13
Functional Materials Design, Discovery and Development Research Group (FMD3),
Advanced Membranes and Porous Materials Center, Division of Physical Sciences
and Engineering, King Abdullah University of Science and Technology (KAUST),
Thuwal 23955-6900, Kingdom of Saudi Arabia.
E-mail: mohamed.eddaoudi@kaust.edu.sa
† Electronic supplementary information (ESI) available: Materials and methods,
elemental analyses, TGA, 1H and 13C NMR, FTIR spectroscopy, gas sorption
isotherms, pore size distribution. See DOI: 10.1039/c3cc48228f
Herein, we report access to amine functionalized POPs using
a novel one-step amine grafting PSM procedure, performed
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under relatively mild synthetic conditions, allowing the conversion moieties (Fig. S4, ESI†). The anticipated high thermal stability
of deliberately introduced free aldehyde moieties, integrated in the of 1 after EtOH exchange (up to ca. 300 1C) was confirmed by
MBB prior to the assembly process, into amine functionalities. To thermogravimetric analysis (TGA) (Fig. S3a, ESI†).
date, aldehyde moieties have been widely utilized for the construc-
In order to assess the permanent porosity of 1, a nitrogen
tion of POPs, particularly in combination with amines or hydrazides sorption isotherm was collected at 77 K, and was found to
to afford the formation of imines or hydrazones groups as links exhibit a type IV like isotherm with H2 type desorption hysteresis,
between the associated MBBs.14 Whereas, our envisioned approach following the IUPAC classification. However, the type IV like
is to preserve aldehyde moieties unreacted during the POP assembly behaviour with H2 hysteresis loop type is not indicative of the
process and to deliberately introduce free aldehyde moieties in the presence of mesoporosity but is most likely associated with the
resultant POP structure. Accordingly, we elected a distinct synthetic occurrence of swelling, commonly observed in these types of
method, namely Sonogashira–Hagihara coupling (SH), in order to structures (Fig. S5a, ESI†).16 The estimated apparent Brunauer–
construct new POPs with an anchored aldehyde functionality. The Emmett–Teller (BET), Langmuir surface areas and total pore
SH reaction permits the coupling of a given halogen (e.g. bromine) volume are 630 m2 gÀ1, 730 m2 gÀ1 and 0.54 cm3 gÀ1, respectively,
and alkyne functionalities from complementary and distinct MBBs and are in the range of other related POPs.11b,15
with the subsequent conservation of the aldehyde groups not
As stated previously, aldehydes have been purposely incorporated
involved in the reaction process. Indeed, the reaction between the as an integral part of the elected MBB and subsequently transposed
two elected MBBs, enclosing the needed functionalities for the SH into 1, allowing a deliberate amine grafting (aldehyde conversion to
coupling, namely, the tri-connected 1,3,5-triethynylbenzene imine) via a mild one-step PSM process. In a typical experiment, 1
(TEB) and the bridging 2,6-dibromo-4-trimethylsilylbenz- is soaked in ethylenediamine (EDA) at 85 1C for three days (see ESI†)
aldehyde (Br2TMSBA) afforded the formation of a dark brown to obtain {[(C2)3–C6H3]2[(CH3)3Si–C6H2–CHNC2H4NH2]3}n (2), subse-
powder, {[(C2)3–C6H3]2[(CH3)3Si–C6H2–CHO]3}n POP (1) (see Fig. 1a quently isolated by Bu¨chner type filtration (Fig. 2a). No obvious colour
and the ESI† for experimental details). Unlike POPs obtained by change was observed and as intended, the full conversion of the
condensation reactions10b or reversible self-addition polymeriza- aldehyde in 1 to the imine moiety in 2 was confirmed from the FTIR
tion,10a POPs formed through SH coupling are widely amorphous,6 data whereby the characteristic nCQO band corresponding to the
making their characterization particularly challenging, as exempli- aldehyde at 1700 cmÀ1 disappeared. (Fig. S4, ESI†).
fied by Cooper and co-workers who contributed significantly to the
field and detailed specific parameters that govern the formation and porosity of the POP, nitrogen sorption was performed on 2 at
pore size control in amorphous POPs.11b,15
77 K and showed, as expected, a similar type IV like isotherm with H2
The successful assembly of the two selected MBBs into 1 was type desorption hysteresis (Fig. S5b, ESI†).16 The estimated apparent
evidenced by Fourier-transformed infrared (FTIR) spectroscopy, BET, Langmuir surface areas and total pore volume are 485 m2 gÀ1
To confirm that our grafting strategy did not overly alter the
,
with the characteristic nCQO band (1700 cmÀ1) attributed to the 525 m2 gÀ1 and 0.39 cm3 gÀ1, respectively. Not surprisingly, these
aldehyde group from TMSBA moieties and the vibration mode values are ca. 25% lower compared to 1, which is consistent with both
at 2160 cmÀ1 corresponding to the CRC bond from the TEB (i) the conversion of the aldehydes to a relatively heavier moiety (15%
of total molecular weight) and (ii) the reduction in the pore size due to
the presence of bulkier amine groups (Fig. S6b and S7b, ESI†). The
isosteric heat (Qst) of CO2 adsorption is a central parameter that
dictates the affinity of the porous sorbent toward CO2, which in turn
plays a major role in determining the adsorption selectivity and the
necessary energy to release CO2 during the regeneration step. Qst of
CO2 adsorption are scarcely reported for POPs and are usually in the
25–15 kJ molÀ1 range (for non-functionalized materials) and mainly
driven by the effect of pore size.9b,13
The CO2 sorption data for 1 and 2 at low pressures verified the
projected improvement after PSM (Fig. 2b and c). Indeed, the Qst
at low coverage (Fig. 2c), determined from isotherms collected at
258, 273, 288 and 298 K (Fig. S6a and S7a, ESI†), increased
substantially from 33 kJ molÀ1 for the pristine 1 to 50 kJ molÀ1
for the functionalized 2, and thus, unambiguously confirming
the great efficiency of our amine grafting strategy on aldehyde
anchors. It should be mention that due to the reversible nature of
imine bonds, prolonged exposure to liquid water may lead to the
resultant POP hydrolytic degradation. However, limited exposure
to water vapour, relevant conditions to industrial process, doesn’t
alter the imine bonds and thus no specific precautions were taken
Fig. 1 (a) Schematic to illustrate the synthetic route to aldehyde containing
to avoid contact with humidity from air prior to the sorption
measurements presented herein.
POPs and (b) structural model; one main motif is highlighted, carbon, oxygen,
silica, hydrogen are represented in grey, red, yellow and white, respectively.
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Fig. 3 CO2 selectivity over N2 and CH4 in CO2/N2:10/90 and CO2/CH4:5/95
mixtures on 1 and 2.
1 bar in the case of CO2/CH4:5/95 and CO2/N2:10/90 gas mixtures,
respectively (Fig. 3), confirming the high potential of our unique
functionalization method using EDA. Notably, this finding is in
excellent agreement with the enhanced CO2 energetics determined
from the single gas variable-temperature CO2 adsorption data.
Although the heterogeneity of 1 and 2 may induce non-ideality, thus
non-negligible prediction errors, the main purpose of the IAST
prediction (Table 1) is to show the enhancement in selectivity
induced by the improved energetics at equilibrium.
In summary, we reported herein for the first time the deliberate
synthesis of a POP with exposed aldehyde functionalities. Subse-
quently, the free aldehyde groups were introduced and employed as
anchors for a unique one-step post-synthetic amine functionalization
strategy. As envisioned, the intended substitution of aldehyde groups
by EDA moieties leads to (i) a substantial increase in Qst (i.e. from 33
to 50 kJ molÀ1 at low coverage), which is characteristic of an enhanced
framework of –CO2 interactions, and (ii) a relatively enhanced quali-
tative CO2/N2 and CO2/CH4 selectivity. Remarkably, regeneration of
the material does not require heating, which is a desired condition for
the next generation of separation agents and therefore paves the way
for further studies using our unique amine functionalization strategy.
Finally, this strategy is not constrained to organic materials, but in fact
can easily be transposed to MOF chemistry.
Fig. 2 (a) Schematic representation of the amine functionalization pro-
cess of 1, (b) comparison of CO2 uptake at low pressure in 1 and 2 (298 K)
and (c) isosteric heats of adsorption of CO2 for 1 and 2.
In contrast to the classical amine supported materials, no heating
was necessary to reactivate 2 between variable temperature CO2
adsorption measurements, thus suggesting potentially reduced
energy requirements for adsorption–desorption cycles. The CO2
sorption uptakes (298 K) at 0.15 and 1 bar were 0.15 mmol gÀ1
,
Table 1 Comparison of CO2/N2 selectivity and Qst for CO2 in 1, 2 and
0.27 mmol gÀ1 and 0.78 mmol gÀ1, 0.95 mmol gÀ1 for 1 and 2
respectively. Preliminary, in situ FTIR experiments did not show the
formation of stable carbamates, therefore it is difficult to deduce the
exact nature of the interactions, i.e. chemisorption or physisorption.
In order to assess the potential of the resultant compounds
as prospective separation agents, CO2, N2 and CH4 adsorption
isotherms at 298 K were recorded within a wide range of
pressure for 1 (Fig. S8, ESI†) and 2 (Fig. S9, ESI†). As expected,
the CO2 selectivity, estimated using ideal adsorption solution
theory (IAST, see ESI†) increased from 5 and 14 for the parent
some of the most promising POPs for CO2 capture
IAST CO2/N2 selectivity
(1 bar, 298 K)
CO2 Qst
Sorbent
(kJ molÀ1
)
Ref.
1
2
14a
33
50
—
25
56
56
32
This work
This work
8a
8b
13
13
8c
155a
25b
COP-1
Azo-COP-2
PPN-6-CH2EDA
PPN-6-CH2DETA
FCTF-1-600
131b
115b
442b
19a/152a,c
a
b
c
CO2/N2 = 10/90. CO2/N2 = 15/85. Determined from breakthrough
POP (1) to 60 and 155 for the amine functionalized POP (2) at measurement.
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