Porous macromolecular dihydropyridyl frameworks exhibiting catalytic and halochromic activity

Article abstract of DOI:10.1039/c4ta02521k

New porous macromolecular frameworks (PMFs) have been designed and prepared by the condensation of dialdehydes with aminoacrylonitriles. Two porous materials were prepared by reacting 3,3′-benzene-1,4-diylbis(3-aminoprop-2-enenitrile) with benzene-1,4-dic

Full text of DOI:10.1039/c4ta02521k

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Porous macromolecular dihydropyridyl
frameworks exhibiting catalytic and halochromic
Cite this: DOI: 10.1039/c4ta02521k
activity†
Bo Xiao,ab Timothy L. Easun,a Amarajothi Dhakshinamoorthy,cg Izabela Cebula,de
Peter H. Beton,d Jeremy J. Titman,a Hermenegildo Garcia,c K. Mark Thomasf
a
*
and Martin Schroder
¨
New porous macromolecular frameworks (PMFs) have been designed and prepared by the condensation of
dialdehydes with aminoacrylonitriles. Two porous materials were prepared by reacting 3,30-benzene-1,4-
diylbis(3-aminoprop-2-enenitrile) with benzene-1,4-dicarbaldehyde and biphenyl-4,40-dicarbaldehyde to
give PMF-NOTT-1 and PMF-NOTT-2, respectively. Adsorption and desorption studies of N2 (77 K) and
CO2 (273–303 K and 20 bar) were used to characterize the porosity of these materials. CO2 adsorption
measurements indicate that these PMFs have similar porosity with Dubinin–Radushkevich micropore
volumes of 0.142 and 0.144 cm3 gꢀ1 and CO2 surface excess uptakes of 28.4 and 28.9 wt% at 20 bar,
273 K for PMF-NOTT-1 and PMF-NOTT-2, respectively. The isosteric heats of adsorption for CO2 at zero
surface coverage were 31.9 kJ molꢀ1 (for PMF-NOTT-1) and 33.1 kJ molꢀ1 (for PMF-NOTT-2). However,
N2 adsorption studies at 77 K indicated that PMF-NOTT-2 shows activated diffusion effects due to the
presence of some narrow ultramicroporosity. The conjugated systems of these frameworks can be
reversibly switched by varying proton concentration in solution and these materials thus demonstrate
halochromic properties. PMF-NOTT-1, constructed from shorter building blocks than PMF-NOTT-2,
exhibits higher catalytic activity and selectivity in Knoevenagel condensation reactions of malononitrile
with benzaldehydes. The advantages of using PMFs as catalysts or adsorbents are their excellent thermal
and chemical stabilities and they can be recovered and regenerated for re-use.
Received 20th May 2014
Accepted 21st August 2014
DOI: 10.1039/c4ta02521k
www.rsc.org/MaterialsA
since degradation of porous structures reduces their activity
and results in low efficiency and poor economics for material
Introduction
utilization. Considerable efforts have been focused on devel-
oping strategies to prepare organic framework materials
showing high chemical stability and permanent porosity.12–16
Diverse and distinct chemical functionalities such as catalyti-
cally active N-heterocyclic moieties can be incorporated into
these materials to formulate catalytic or optoelectronic prop-
erties.17,18 In previous work, we have conrmed the construction
of a supramolecular organic framework (SOF) via condensation
of aminoacrylonitrile with terephthalaldehyde to produce 1,4-
bis(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (3 in
Scheme 1).19 This material packs in the solid state to afford a
stable three dimensional framework held together via hydrogen
bonds and p–p stacking interactions. The framework shows
permanent porosity and effective gas storage properties.
Porous materials fabricated by the assembly of molecular
building units have been extensively investigated, especially for
applications for gas storage and capture, and catalysis.1–11 Such
materials need to show both chemical and physical stability
aSchool of Chemistry, University Park, University of Nottingham, Nottingham NG7
2RD, UK. E-mail: m.schroder@nottingham.ac.uk; Fax: +44 (0)115 9513563; Tel: +44
(0)115 9513490
bSchool of Chemistry & Chemical Engineering, Queen's University Belfast, David Keir
Building, Stranmillis Road, Belfast BT9 5AG, UK
c
´
´
Instituto de Tecnologıa Quımica (CSIC-UPV), Avda. de los Naranjos s/n., 46022
Valencia, Spain
dSchool of Physics and Astronomy University Park, University of Nottingham,
Nottingham NG7 2RD, UK
eInstitute of Experimental Physics, University of Wroclaw, pl. M. Borna 9, 50-204
We report herein a “bottom up” strategy based on the
condensation of 1,4-phenylaminoacrylonitrile 4 with 1,4-phe-
nyldialdehydes 5 to generate porous macromolecular frame-
works (PMFs) 6, denoted as PMF-NOTT-n (n ¼ 1 or 2) (Scheme
1). These porous macromolecular frameworks contain dihy-
dropyridine groups, which can be protonated, and this exerts
a signicant impact on electron delocalization in these
Wroclaw, Poland
fWolfson Northern Carbon Reduction Laboratories, School of Chemical Engineering
and Advanced Materials, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
gCentre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj
University, Tamil Nadu, India, 625 021
† Electronic supplementary information (ESI) available: Synthetic details, solid
state NMR, IR and UV-Vis spectra, SEM, gas adsorption analysis, and catalytic
reaction. See DOI: 10.1039/c4ta02521k
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3.2, N 15.6%; for PMF-NOTT-2 (C19H10N3) calculated: C 81.4, H
3.6, N 15.0%; found: C 83.3, H 3.5, N 13.2%.
Stability tests. PMF-NOTT-1 and PMF-NOTT-2 were tested in
the following solvents: n-hexane, toluene, CH2Cl2, tetrahydro-
furan (THF), CHCl3, ethyl acetate, methanol, MeCN, DMF and
dimethyl sulfoxide (DMSO). Tests were carried out at ambient
temperature and the samples were placed in solvents for two
1
weeks. The yellow PMFs did not dissolve as conrmed by H
NMR spectroscopy of the CDCl3 or DMSO-d6 solvent. The PMFs
did not dissolve in acids (H2SO4, HNO3 and HCl, 2–6 M) at room
temperature, or in bases (NaOH and KOH, 2–6 M). In basic
solution, they changed from yellow color (protonated form) to
red (deprotonated form). The red deprotonated PMFs can be re-
protonated to the yellow form by water or acids.
Scheme
1 Synthesis of (a) molecular building block 3 for SOF
Characterization of PMF materials
formation and (b) porous macromolecular frameworks 6.
Infrared spectroscopy. Thermo Scientic Nicolet iS5 with iD
5 Diamond Advanced ATR was used to collect IR spectra of
powder samples, with 16 scans per spectrum and a resolution of
amine-functionalized conjugated systems thereby imparting
halochromism to the porous polymer. In this study we show
that the halochromism can be used to follow the course of
catalytic Knoevenagel condensation reactions of malononitrile
with benzaldehydes.
4 cmꢀ1
.
Thermogravimetric analysis (TGA). Thermogravimetric
analysis was conducted using a PerkinElmer Pyris 1 TGA ana-
lyser. The weight loss proles of samples with increasing
temperature were obtained at a heating rate of 3 K minꢀ1 under
a ow of N2.
Experimental
Synthesis of porous macromolecular frameworks (PMFs)
Solid-state NMR. 13C CPMAS spectra were recorded using a
Bruker Advance III spectrometer operating at a 13C Larmor
Synthesis of 3,30-benzene-1,4-diylbis(3-aminoprop-2-eneni- frequency of 150.9 MHz and equipped with a triple-resonance
trile). In a typical synthesis, 1,4-dicyanobenzene (2.6 g, 20 2.5 mm MAS probe. Cross-polarization was achieved using a
1
mmol) was mixed with acetonitrile (MeCN) (3.3 g, 80 mmol) in ramped H contact pulse and decoupling using the SPINAL64
benzene (200 mL) until all solids were dissolved. Potassium t- sequence with a radio frequency eld of 100 kHz. The MAS rate
butoxide (13.5 g, 120 mmol) was added to the solution to form was 17 kHz, the contact time was 2 ms, the relaxation delay was
an orange suspension, which was stirred continuously at 2 s, and 20 000 scans were acquired. Chemical shis were
ambient temperature for 48 h. An aqueous saturated solution of referenced externally to TMS using adamantane as a secondary
NaHCO3 (500 mL) was added and the orange solid was collected reference. Assignments were made with the help of the 13C
and dissolved in hot dimethylformamide (DMF) (100 mL). The structure prediction package available as part of the Marvin-
solution was concentrated by rotary evaporation to remove Sketch soware (ChemAxon Ltd.)
ꢁ80% of the DMF and CH3OH–water (300 mL; 1 : 5 (v/v)) was
UV-Vis spectra. Solid-state UV/Vis spectra were recorded
added to give a white precipitate. The white solid product was using an Ocean Optics USB2000+UV-VIS-ES spectrometer
collected and dried under vacuum. The product yield was 50– equipped with a DT-MINI-2-GS light source and an R400-7-UV-
70%. IR (ATR) n ¼ 3438, 3333, 3244, 2188, 1637, 1578, 1513, VIS ber-optic reection/backscattering probe. Spectra were
1422, 1292, 853, 794, 695, 607 cmꢀ1 (see Fig. S2†); 1H-NMR recorded using Spectrasuite soware, averaging 10 scans with
(DMSO-d6): 7.64 (s, 4H, H-Ar), 6.88 (s, 4H, –NH2), 4.27 ppm (s, an accumulation time of 500 s per scan.
2H, ]CH–CN) (see Fig. S1†).
Adsorption studies. A Hiden Intelligent Gravimetric Ana-
Syntheses of PMF-NOTT-1 and PMF-NOTT-2. 3,30-Benzene- lyser (IGA) was used to measure CO2 adsorption isotherms.
1,4-diylbis(3-aminoprop-2-enenitrile) (1.26 g, 6.0 mmol) was The IGA is a fully computer controlled microbalance in an
mixed with benzene-1,4-dicarbaldehyde (0.40 g, 3.0 mmol) or ultra-high vacuum (UHV) system and has both pressure and
biphenyl-4,40-dicarbaldehyde (0.63 g, 3.0 mmol) in glacial acetic temperature regulation systems. The microbalance had a
acid (50 mL). The solution was reuxed at 120 ꢂC for 24 h until a long-term stability of ꢃ1 mg with a weighing resolution of 0.2
yellow precipitate was formed. The hot suspension was ltered, mg. Activation of samples was carried out by heating to 393 K
and the yellow solid product collected, washed thoroughly with under vacuum (<ꢁ10ꢀ4 mbar) until desorption was complete
water until the pH of the washings was ꢁ7.0. The yellow product (typically for ꢁ6–12 h). The pressure transducer used had a
was washed with CH3OH via Soxhlet extraction to remove range of 0–2 MPa. Adsorption isotherms for CO2 up to 20 bar
residual impurities and the material was dried in vacuo to give at 273–303 K were obtained using a circulating water–
the PMFs in about 65% yield. Analytical data: for PMF-NOTT-1 ethylene glycol bath to control the sample temperature. The
(C16H8N3) calculated: C 79.3, H 3.3, N 17.3%; found: C 81.2, H pressure and temperature were controlled by computer using
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IGA soware during isotherm measurements. N2 and H2
adsorption isotherms were obtained on a Quantachrome
Autosorb instrument with the adsorption pressure set in a
range of 0–1 bar. H2 adsorption isotherms were obtained at
two adsorption temperatures 77 (liquid nitrogen) and 87 K
(liquid argon). Before each adsorption experiment, the
sample was degassed under vacuum at 383 K. N2 adsorption
at 77 K was used to determine BET surface areas. Two optimal
batches of products were compared, and the BET surface
areas varied within the ranges of 410–470 and 43–60 m2 gꢀ1
for PMF-NOTT-1 and PMF-NOTT-2, respectively. For conve-
nience, the adsorption analysis was based on the same batch
of each PMF product. CO2 adsorption at 273 K was used to
determine micropore surface areas and volumes, and non-
local density function theory (NLDFT) with a slit shaped pore
model was used to t the adsorption branch of the N2
adsorption/desorption isotherm (77 K) to obtain pore size
distributions. The CO2 adsorption isotherms were analyzed
using the virial eqn (1):
Results and discussion
Determination of PMF structures and solvent and thermal
stabilities
The synthesis of PMF-NOTT-1 and PMF-NOTT-2 requires only
relatively mild reaction conditions involving glacial acetic acid
ꢂ
at 120 C rather than complex metal catalysts as required, for
example, for the synthesis of poly(aryleneethynylene) via Pd-
catalyzed Sonogashira–Hagihara cross-coupling20 or of poly-
carbazoles via FeCl3 catalyzed oxidative coupling.21 In compar-
ison, the adopted synthetic route described herein is much
simpler and therefore, potentially cheaper for scaling up. The
PMF materials show high chemical stability for use as catalysts
in synthetic environments and adsorption in aqueous systems.
The yellow solid PMF powder products obtained are insoluble
in common solvents such as n-hexane, toluene, CH2Cl2, THF,
CHCl3, ethyl acetate, methanol, acetonitrile, DMF and DMSO,
acids (2–6 M of H2SO4, HNO3 and HCl) and bases (2–6 M of
NaOH and KOH) at ambient temperature, and are generally
chemically robust. The following catalytic reactions operated at
ln(n/P) ¼ A0 + A1n + A2n2 +.
(1)
ꢂ
110 C in toluene also indicated the relative stability of these
PMF materials. Such resistance toward solvents and acids/bases
has been observed for other nanoporous covalent organic
frameworks (COFs) such as conjugated microporous polymers
(CMP-1 ꢁ 4)20 and porous polymer networks (PPN-1 ꢁ 5)22,23
fabricated through various coupling reactions. As potential
catalysts, it is crucial that the PMFs have high chemical stability
in various media without reduction in catalytic activity. This
property distinguishes PMFs from many metal organic frame-
work (MOF) materials, which, although having unique micro-
porous structures, can be unstable in aqueous solution or in
humid gas, thus incurring extra expense in drying the feed
stream before contacting with MOFs.
where P is pressure (Pa), n is adsorption quantity (mol gꢀ1) and
An are virial coefficients. At low uptakes, a plot on ln(n/P) versus
n is linear with gradient A1 and intercept A0. A0 is exp(KH), where
KH is the Henry's law constant. The isosteric adsorption heat at
zero coverage is expressed as
vA0
qst;0 ¼ R
(2)
vð1=TÞ
where R is gas constant (8.314 J Kꢀ1 molꢀ1) and T is the
adsorption temperature (K). A plot of A0 versus 1/T has a
gradient of qst,0/R and this was used to calculate qst,0
.
The core structures of PMF-NOTT-1 and PMF-NOTT-2 were
identied by IR and solid-state 13C NMR spectroscopies. 13C
CPMAS NMR spectra at the MAS rate of 17 kHz demonstrate
(Fig. 1) the similarity between the structures of the two mate-
rials. The intense peaks between 140 and 127 ppm are assigned
Knoevenagel condensation reactions using PMFs as
organocatalysts
A 50 mL round-bottomed ask was charged with the catalyst (20
mg) in toluene (4 mL), aldehyde (1 mmol) and malononitrile (1
ꢂ
mmol). The reaction mixture was stirred at 110 C for ꢁ55 h.
The reaction was monitored periodically by analyzing the
sample using a Hewlett Packard 5890 series II gas chromato-
graph (GC) with a FID detector and high purity He as the carrier
gas. The products were identied by gas chromatography-mass
spectrometry (GC-MS) using a Hewlett Packard 6890 series
spectrometer and by comparison with pure authentic
commercial samples. The yields of the product were determined
by using nitrobenzene as the external standard considering the
response factors as unity. The mass balances of the recovered
reaction mixture accounted for more than 96% of the initial
substrate. Aer the catalytic reaction, the catalyst was recovered
by ltration, washed in acetonitrile and dried under air at room
temperature. The recovered catalyst was used for the next run
with fresh aldehyde and malononitrile. The percentage
conversion, purity and relative yields of the nal products were
determined.
Fig. 1 13C CPMAS NMR spectra of PMF-NOTT-1 and PMF-NOTT-2
recorded at a Larmor frequency of 150.9 MHz and a MAS rate of 17
kHz. Spinning sidebands are indicated by asterisks.
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to the aromatic carbon centers, except for the quaternary carbon
The thermal stability of PMFs has been assessed by ther-
5, which contributes to the resolved shoulder at 148 ppm, along mogravimetric analysis (TGA) which conrms that both are
with carbon 4. The weaker peaks at 118, 84 and 43 ppm corre- thermally stable up to about 400 ꢂC (Fig. 3). Importantly, IR
spond to –CN (carbon 3), –C]C– (carbon 2) and the aliphatic spectra of the heat-treated samples (400 ꢂC for 30 minutes)
carbon on the dihydropyridyl group (carbon 1), respectively. The conrm that the cyano groups remain intact. Above this
presence of the latter resonance conrms that the structure temperature, the PMFs are gradually carbonized, but without
retains a dihydropyridyl ring, rather than the aromatic pyridyl such rapid decomposition as oen found with carboxylate
alternative. IR spectra show (Fig. 2) sharp intense peaks at metal organic frameworks (MOFs).27 The TGA analyses
ꢁ2204 cmꢀ1 characteristic of a n(C^N) stretching vibration. conrmed that PMF-NOTT-1 has weight losses of 11.2 and
These results conrm that the PMFs have extended networks 14.7 wt% at 100 and 400 ꢂC, respectively. The corresponding
with a core structure analogous to molecule 3. The functional- values for PMF-NOTT-2 were 6.8 and 7.9 wt%. Most of the water
ities (–CN and –NH–) not only affect the conjugated system of adsorbed in PMFs is released at low temperatures below
ꢂ
PMFs, but also provide opportunities for further functionali- ꢁ100 C, and the amounts of water released in the thermogra-
zation of materials. These functionalities also endow the PMFs vimetric studies of PMF-NOTT-1 and PMF-NOTT-2 are, to some
with specic catalytic activity. Powder X-ray diffraction proles extent, related to the porosity of PMFs.
for PMF-NOTT-1 and PMF-NOTT-2 do not show any signicant
peaks conrming that the PMF structures are amorphous.
Irregular cross-linking and disordered interpenetration might
exist in the polymer structures, in particular, for PMF-NOTT-2
which is constructed from long building molecular struts. The
in solution pH, reversibly switching color between yellow and
lack of information about the long range order or connectivity
of molecular building blocks makes structural modeling prob-
ochromism is attributed to the dihydropyridine moieties
lematic. This is a wide spread problem for this type of a polymer
material. Elemental analysis shows discrepancies between the
experimental and calculated values based on ideal PMF struc-
Halochromic property of PMFs
These chemically robust PMF solids are responsive to changes
red at low and high pH, respectively (Fig. 4). Such notable hal-
embedded in the framework. The reversible deprotonation and
protonation of dihydropyridines exerts a signicant impact on
electron delocalization in this amine-functionalized conjugated
system, as revealed by changes in UV-Vis absorption spectra
tures. Similar differences have been reported previously for
porous organic polymers, for example CMP-0 ꢁ 2,24 pp-CMP,25
COF-1 ꢁ 5 4 and TFPT-COF.26 For both PMF-NOTT-1 and PMF-
(Fig. S3†). The protonated forms of the PMFs show broad
absorption below 420 nm resulting in yellow solids. In alkaline
NOTT-2, the experimentally determined carbon content is
solution, the PMFs are deprotonated and the broad absorption
bands shi signicantly to lower energies with the polymers
changing to red in color. These shis in absorption to longer
wavelength and lower energy are consistent with an extension of
the conjugated p system resulting in a decrease in the HOMO–
LUMO energy gap. Conversely, lowering the pH acts as a “switch
higher and nitrogen analysis is lower than expected. Therefore,
it is possible that a higher proportion of phenyl groups may be
present in PMF structures than expected due to a small amount
of unreacted and trapped phenyl aldehyde compound 5. Only
one aldehyde group participates in cyclisation reaction to form
a
dihydropyridine node, and minor amounts of these
off” function by protonating the pyridine moiety and reducing
the conjugated p system. Therefore, these solvent-insoluble
halochromic PMF solid particles change color reversibly and
have potential as specic pH indicators or specic dyes with a
characteristic switching ability.
substructures may inuence the PMF analysis, structure,
adsorption characteristics and catalytic properties. Purication
and recrystallization of these materials is further inhibited by
their insolubility in common solvents.
Fig. 3 Thermogravimetric weight loss profiles of PMF-NOTT-1 and
Fig. 2 Solid-state ATR-IR spectra of PMF-NOTT-1 and PMF-NOTT-2. PMF-NOTT-2.
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Fig. 4 Reversible responses of PMFs to pH changes (a and d): fresh PMF/Polytetrafluoroethylene (PTFE) disks; (b and e): 2 M NaOH treatment of a
and d; (c and f): 2 M HCl treatment of b and e.
the D–R micropore volume. The D–R micropore volumes for
CO2 adsorption isotherm data at 273 K were measured as
ꢁ0.142 ꢃ 0.001 and 0.144 ꢃ 0.001 cm3 gꢀ1 assuming a CO2
density of 1.032 g cmꢀ3 for PMF-NOTT-1 and PMF-NOTT-2,
respectively. The CO2 micropore volumes for both materials
correspond to a surface area of ꢁ380 m2 gꢀ1 (assuming area of
CO2 molecule ¼ 1.9 ꢄ 10ꢀ19 m2). This is similar to the N2 (77 K)
BET surface area (414 m2 gꢀ1) for PMF-NOTT-1. It also conrms
Characterization of porosity
Both PMFs have extensive porous structures and adsorb N2 and
CO2. PMF-NOTT-1 has a much higher BET surface area (ꢁ414
m2 gꢀ1) than PMF-NOTT-2 (ꢁ60 m2 gꢀ1) as determined by N2
adsorption at 77 K (Fig. S4†) revealing a high concentration of
accessible micropores in PMF-NOTT-1. This contrasts with the
CO2 adsorption data discussed later, which shows that the
isotherms and micropore volumes for PMF-NOTT-1 and PMF-
NOTT-2 are very similar (Fig. 5 and S6†). The relatively low value
of the BET surface area for PMF-NOTT-2 is ascribed to N2 acti-
vated diffusion in very narrow ultramicroporosity possibly
formed by the irregular cross-links and disordered interpene-
tration within the structure. Non-local density functional theory
(NLDFT) analysis conrms the existence of two types of pores in
PMF-NOTT-1 both in the ranges of ꢁ<1.7 nm and ꢁ2.5–5.0 nm,
corresponding to micropores and mesopores, respectively
(Fig. S5†). The activated diffusion effects in PMF-NOTT-2 lead to
the contribution from the narrowest porosity being reduced in
the pore size distribution. The presence of mesoporosity is
thought to be responsible for the characteristic hysteresis
observed in both the N2 and CO2 isotherms, and indeed mes-
oporosity facilitates rapid transport during adsorption and
catalysis.
The type I isotherms observed for CO2 adsorption (Fig. 5) up
to 20 bar at 273 K conrm surface excess uptakes of ꢁ28.4 and
28.9 wt% at 20 bar (ꢁ6.5 mmol gꢀ1 at p/p0 ꢁ 0.57) for PMF-
NOTT-1 and PMF-NOTT-2, respectively. The Dubinin–Radush-
kevich (D–R) plots (Fig. S6†) are linear for the low relative
pressure range and deviate upwards at higher relative pressures.
This indicates the presence of larger pores sizes consistent with
the pore size distribution obtained from N2 adsorption at 77 K
(Fig. S5†). Extrapolation of the linear region to give the intercept
at ln2(p/p0) ¼ 0 provides the amount adsorbed corresponding to
Fig. 5 Adsorption isotherms of CO2 gas on PMF-NOTT-1 and PMF-
NOTT-2 (solid: adsorption; open: desorption).
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that both PMFs have similar micropore volumes and surface molecular sieves (28.4 ꢃ 0.7 kJ molꢀ1).29 A wide range of qst,0
areas accessible to CO2 at temperatures of 273 K and above. The values have been obtained for CO2 binding in MOFs ranging
Langmuir plots for CO2 adsorption at 273 K were linear (R > from 20–96 kJ molꢀ1 with H3[(Cu4Cl)3(BTTri)8] (H3BTTri ¼
35
0.998) in the region 10–20 bar and extrapolation to the saturated 1,3,5-tri(1H-1,2,3-triazol-4-yl)benzene)) incorporating N,N0-
vapor pressure (34.92 bar) gives uptakes of 7.79 ꢃ 0.33 and 7.90 dimethylethylenediamine (mmen) groups exhibiting one of the
ꢃ 0.31 mmol gꢀ1 for PMF-NOTT-1 and PMF-NOTT-2, respec- highest values of qst,0 of 96 kJ molꢀ1
.
The isosteric heat of
36
tively. These values are not signicantly different and corre- adsorption decreases with increasing surface coverage
spond to total pore volumes of 0.332 ꢃ 0.014 and 0.337 ꢃ 0.013 (Fig. S9†). This trend is different from that observed for the
cm3 gꢀ1 for PMF-NOTT-1 and PMF-NOTT-2, respectively. The N2 interpenetrated MOF NOTT-202a, where there is an initial
adsorption data at 77 K gave total pore volumes of 0.385 cm3 gꢀ1 decrease followed by an increase in heat of adsorption.37 The
for PMF-NOTT-1 and 0.042 cm3 gꢀ1 for PMF-NOTT-2 (at p/p0 ¼ difference in the trends is attributed to framework exibility in
0.995, assuming liquid N2 density 0.808 g cmꢀ3). The total pore the latter. Recently, Xie et al. used organic polymer materials
volumes from N2 (77 K) and CO2 (273 K) are comparable for loaded with cobalt-based heterogeneous catalysts to adsorb CO2
PMF-NOTT-1. The lower N2 total pore volume and surface area and then convert propylene oxide into propylene carbonate at
values for PMF-NOTT-2 can be attributed to activated diffusion atmospheric pressure and room temperature.38
effects through very narrow constrictions in the porosity, which
H2 adsorption isotherms for both PMFs are reversible with
is the critical rate determining process at 77 K.
an adsorption capacity of ꢁ0.8 wt% at 0.1 MPa at 77 K
In terms of the pore size distributions, the upward curvature (Fig. S10†), which is similar to that of nanoporous polyani-
of the D–R plots (Fig. S6†) are an indication of available lines,39 but lower than the porous poly(aryleneethynylene) (PAE)
adsorption volume being lled beyond that predicted by the polymers.24 Both PMFs show isosteric heats of adsorption qst,0 at
Gaussian micropore distribution function used in the deriva- zero coverage for H2 uptake of about 8.9–9.5 kJ molꢀ1 (Fig. S11†)
tion of the D–R equation. This “additional” adsorption capacity which is comparable to those values of the CMP polymers (CMP-
could be due to (a) lling of supermicroporosity, (b) lling of 0-5: ꢁ10.0 kJ molꢀ1) but higher than the hypercross-linked
mesoporosity or (c) multilayer formation. In the case of the polystyrene polymers40 and most MOFs that do not contain
materials described herein, mesopores are being lled. Other unsaturated metal centers (4.0–7.0 kJ molꢀ1).41–44 In addition to
investigations have conrmed that adsorption in polymers of the microporous structure providing strong overlap potentials,
intrinsic microporosity (PIMs) and conjugated microporous the diverse polar functionalities in these materials, most
polymers (CMPs) may involve potential network swelling and notably N–H and CN groups, may increase host–guest surface
hysteretic adsorption.12–16,28
interactions.
The CO2 isotherms can be described by virial eqn (1). The
values of A1 for both materials were similar for tting the
isotherm data to a two-parameter linear virial equation at low
Catalytic studies
uptake and a three-parameter virial equation for the complete The catalytic performances of PMF-NOTT-1 and PMF-NOTT-2
isotherm data (Fig. S7†). The values of A1 from the linear virial were assessed in Knoevenagel condensation reactions (Fig. 6).
plots for 273–303 K lay in the range ꢀ972 to ꢀ1179 g molꢀ1 and Both materials show high selectivity (typically 98%) conrming
ꢀ855 to ꢀ1057 g molꢀ1 for PMF-NOTT-1 and PMF-NOTT-2, good catalytic specicity with little by-product formation. The
respectively, and describe the CO2–CO2 interactions (Fig. S7†). lower conversion using PMF-NOTT-2 is ascribed to restrictions
These values are very similar to the values of A1 (ꢀ1000 to ꢀ1045 imposed on reactants entering micropores where active sites are
g molꢀ1 for 303–343 K) observed for CO2 adsorption on a carbon concentrated, as discussed above. A similar case has been
molecular sieve (CMS) material used for air separation by observed by Kitagawa et al.45 in a MOF, where amide groups,
pressure swing adsorption.29 The CMS material was prepared by which act as weak base active sites, are arranged facing the open
carbon deposition on an ultramicroporous carbon substrate to channels. The reduction of porosity in the desolvated MOF is
provide kinetic selectivity for molecular sieving of oxygen and proposed to restrict the interaction of larger substrates with
nitrogen.29–31
sites inside channels, resulting in lower conversion.45 Notably,
The values of A0 for PMF-NOTT-1 and PMF-NOTT-2 were at the same selectivity of ꢁ98%, incorporation of –CH3 groups
used to determine the isosteric adsorption heats at zero into the aldehyde substrate leads to conversions of ꢁ93%,
coverage (qst,0) from eqn (2), which is a fundamental measure of signicantly higher than for –Cl and –CN derivatives, which are
the CO2–surface interactions. These values were found to be converted in 64% and 54% yields, respectively (Fig. 6 and Table
33.1 ꢃ 3.3 and 31.9 ꢃ 1.4 kJ molꢀ1 for PMF-NOTT-1 and PMF- S1†). The inuence of the –CH3 group on increasing –CHO
NOTT-2, respectively, suggesting that both materials have activity is against the general reactivity of carbonyl groups in
similar surface interactions with CO2 (Fig. S7 and S8†). These condensation reactions based solely on electronic effects46–48
qst,0 values are comparable to those of porous COF materials and probably derives from the higher activity of PMF-NOTT
rich in CO2-binding moieties such as –N]N–, –COOH and catalysts for these substrates, most likely by preferential
–SO3H moieties, for example, azo-COP-1-3 (ꢁ32.1 kJ molꢀ1),32 adsorption and higher affinity for p-tolualdehyde with respect to
CMP-1-COOH (ꢁ32.6 kJ molꢀ1
) and PPN-6-SO3H/SO3Li (30.0– –Cl or –CN substituted benzaldehydes.
33
35.0 kJ molꢀ1).34 These values are higher than those observed
The catalytic stability and activity of PMF-NOTT-1 were
for materials without polar functionalities such as carbon specically tested in the conversion of benzaldehyde to
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Fig. 8 Re-usability test of PMF-NOTT-1 for the reactions between
benzaldehyde and malononitrile in toluene at 110 ꢂC.
piperidine as active components,49 which are effective as cata-
lysts in decarboxylative Doebner–Knoevenagel condensation
reactions, but are soluble in product mixtures and cannot be
recovered for re-use.
Fig. 6 Knoevenagel condensation reactions in toluene at 110 ꢂC.
Selectivity refers to the adduct formation. Yields can be calculated by
multiplying conversion by selectivity.
Conclusions
A new strategy has been developed for the design and prepa-
ration of porous macromolecular frameworks with high chem-
ical stability. The gas adsorption characteristics of these PMF
materials conrm the presence of extensive accessible porous
structures. The framework functionalities have been charac-
terized by spectroscopic methods. The dihydropyridyl moieties
in the framework structure may undergo reversible protonation/
deprotonation and this results in intense halochromic charac-
teristics. The resultant materials exhibit catalytic activity
towards Knoevenagel condensation reactions, their primary
advantages being that they can be recovered for re-use without
signicant loss of catalytic activity and show reversible color
changes, which are useful for monitoring catalytic activity.
benzylidenepropanedinitrile. On removal of the catalyst by
ltration at 15% conversion during the reaction with benzal-
dehyde, no further reaction was observed with conversion
stabilizing at ꢁ15–18% (Fig. 7). This provides direct evidence of
the stability of the active sites within PMF-NOTT-1, indicating
that they are stable and not leached out into solution during
reaction. The reusability of PMF-NOTT-1 was also tested (Fig. 8).
Aer each reaction run, the catalyst was removed by ltration,
washed twice with acetonitrile, dried at room temperature and
then reused for another catalytic run. The reproducibility of
benzaldehyde conversion vs. time indicates that PMF-NOTT-1
has excellent chemical stability, and importantly, it can be
regenerated by ltration followed by acetonitrile washing and
drying in a convenient manner at least three times without
signicant loss of catalytic activity. This is an advantage over
other analogous polymeric catalysts, such as bifunctional
polystyrene bearing 4-dimethylaminopyridine (DMAP) and
Acknowledgements
We thank EPSRC and the University of Nottingham for support.
MS gratefully acknowledges receipt of an ERC Advanced Grant.
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