A nanoporous graphene analog for superfast heavy metal removal and continuous-flow visible-light photoredox catalysis

Article abstract of DOI:10.1039/c7ta05534j

We report a highly recyclable, 2D aromatic framework that offers a unique and versatile combination of photocatalytic activity and heavy metal uptake capability, as well as other attributes crucial for green and sustainable development technologies. The graphene-like open structure consists of fused tritopic aromatic building blocks (i.e., hexahydroxytriphenylene and hexaazatrinaphthylene) that can be assembled from readily available industrial materials without the need for transition metal catalysts. Besides fast and strong binding for Pb(ii) ions (e.g., removing aqueous Pb ions below the drinkable limit within minutes), the alkaline N-heterocycle units of the robust and porous host are able to quantitatively catalyse Knoevenagel reactions in water. Furthermore, the fused donor-acceptor aromatic π-systems enable environmentally friendly photoredox catalysis (PRC) utilizing the safe and abundant visible light in a commercial flow reactor. Also discussed is a new metric for benchmarking the kinetic performance of sorbents in the context of heavy metal removal from drinking water.

Full text of DOI:10.1039/c7ta05534j

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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A nanoporous graphene analog for superfast heavy metal removal
and continuous-flow visible-light photoredox catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Ran Xiao,a,† John M. Tobin,b,† Meiqin Zha,a Yunlong Hou,a Jun He,c Filipe Vilela,b,* Zhengtao Xua,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report a highly recyclable, 2D aromatic framework that offers storage/delivery,5
photocatalytic
H2
production,6
a unique and versatile combination of photocatalytic activity and heterogeneous photocatalysis,7 and other light harvesting
heavy metal uptake capability, as well as other attributes crucial applications.7a, 8
for green and sustainable development technologies. The
graphene-like open structure consists of fused tritopic aromatic to target a single, specific type of application, and a clear gap
building blocks (i.e., hexaoxotriphenylene and exists in the research of versatile PPFs that offer multiple
In spite of the rich functionality, most PPFs were designed
hexaazatrinaphthylene) that can be assembled from readily capabilities for applications.9 For example, even though heavy
available industrial materials without the need for transition metal removal10 and photocatalysis7c, 7d have been separately
metal catalysts. Besides fast and strong binding for Pb(II) ions accomplished in some PPF systems, none has been reported to
(e.g., removing aqueous Pb ions below the drinkable limit within simultaneously address these two key sustainability topics on
minutes), the alkaline N-heterocycle units of the robust and water purification and light harvesting. To promote the
porous host is able to catalyse quantitatively Knoevenagel development of versatile PPFs for green applications, we here
reactions in water. Furthermore, the fused donor-acceptor present an easy-to-make system (HOTT-HATN, Fig. 1) that
aromatic π-systems enable environmentally friendly photoredox features a broad array of green credentials, making for a
catalyses (PRC) utilizing the safe and abundant visible light in a notable example in integrating multiple sustainability criteria
commercial flow reactor. Also discussed is a new metric for into the development of porous materials.
benchmarking kinetic performance of sorbents in the context of
heavy metal removal from drinking water.
Besi
des
clea
ning
up
Due to their large surface areas, functional diversity and robust
reusability, porous polymer frameworks (PPFs) are poised to
address critical topics concerning clean environment,
renewable energy and sustainable chemical processes.1
Compared with coordination networks (aka MOFs),2 the
covalent PPFs offer better stability, and can be conveniently
assembled without the need of crystallization. Besides the
lead
ions
from
wat
er
common uses of PPFs in gas sorption/storage and as catalysis
supports,3 conjugated PPFs,1d,
4
with their extended π-
for energy
and
capt
urin
g
systems,
offer
distinct
advantages
the
safe
and
abu
nda
nt
visib
le
Fig. 1. Synthetic scheme of the honeycomb net of HOTT-HATN (shown
in a ball-stick model; red sphere, O; green, N; grey, C).
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light for the topical photoredox catalysis (PRC), the HOTT- indicating the efficient displacement of the chloro groups.10a
HATN framework is simply assembled from low-cost building Thermogravimetric analysis (TGA, Fig. S4) reveals a stable mass
blocks without using toxic or expensive metal catalysts. In up to 250°C, indicating a good thermal stability comparable to
addition, the alkaline N donors of the polymer backbone also other PPFs.12 The FT-IR peaks (Fig. S5) at 1600 cm-1 and at 1200
enables acid-base catalysis using water as a green reaction and 1250 cm-1 correspond to the C=N and C-O stretches,
medium. Moreover, this robust framework can also be respectively. Solid state 13C NMR CP-MAS analysis (Fig. S6)
deployed as a highly efficient heterogeneous photocatalyst in confirmed the fully aromatic system with four dominant peaks
a commercial flow reactor for optimal separation and recycling at 110.68, 126.27 142.45 and 147.07 ppm, indicating some of
advantages. To our knowledge, such a wide-ranging green- the 7 different carbon atoms of HOTT-HATN were not resolved
chemistry profile remains unprecedented among PPF in this measurement. Solid state UV-Vis analysis of the
materials.
polymer features broad absorbance in the visible spectrum
On the fundamental level, the fused, fully conjugated with a λmax at 460 nm (Fig. S7).
aromatic π-system of HOTT-HATN promotes electronic
PXRD established HOTT-HATN as an amorphous solid (Fig.
communication throughout the net, providing a versatile 2D S8), while SEM and TEM images (Fig. S9, S10) revealed highly
electronic platform reminiscent of graphene. Unlike the all- textured, layer-like morphology. N2 sorption (at 77 K)
carbon graphene system, the O and N heteroatoms here experiments confirm the porous character of HOTT-HATN, with
constitute distinct electron donor-acceptor units. Such D-A a typical type-II N2 gas adsorption isotherm (Fig. 2) revealing a
units generally exhibit intense photoactivities (e.g., for light BET surface area of 526.5 m2·g-1. QSDFT analysis on pore size
harvesting), and were utilized herein to drive photoredox distribution and pore volume showed an average pore width
reactions in quantitative yields. Moreover, the chelating of 0.52 nm and a micropore volume of 0.579 cm3·g-1 (Fig. S11).
pyridinyl N donors on the backbone π-system stand to anchor
Removal of lead from water
various metal species, so as to enrich the optical and electronic
properties for wider applications.
Lead is a cumulative poisonous pollutant that can affect nearly
every system in the body. Children under 6 and pregnant
women are most susceptible to the negative health effects of
lead exposure. The removal of lead and other toxic heavy
metals from water sources therefore remains a high priority in
the public health sector.13 In this regard, HOTT-HATN, with its
dense array of bipyridine donors (e.g., these are commonly
used for metal chelation and catalysis in solution chemistry14)
offers clear potential. Moreover, the HOTT-HATN powder is
freely dispersible in water, which further facilitates green
chemistry applications. Such hydrophilicity can be attributed
to the polar aza and oxo groups built into the polymer grid.
Incidentally, hydrophilicity is rare among PPFs and often has to
be achieved through special synthetic design or modification.15
The metal uptake studies here follow our long-standing
interest in developing metal-binding frameworks for
environmental and catalytic applications.3g, 11a To probe lead
sorption kinetics, a powder sample of HOTT-HATN (20 mg) was
added to an aqueous solution of Pb(NO3)2 (10 ppm; 50 ml) and
stirred at room temperature (rt). The Pb remaining in the
solution was quantified at specific time intervals via ICP-AES.
Within 100 seconds the Pb content already dropped below
0.60 ppm, with over 94% of the total Pb removed by the
polymer sorbent; within 5 minutes, the Pb content was already
reduced beyond the detection limit (15-20 ppb) of the
instrument (Table S2, Fig. S12). Such superfast kinetics is on
par with a thiol-equipped PPF system for Hg removal (e.g.,
removing over 99.8% of heavy metal within 5 min, in similar
conditions regarding the solvent/sorbate/sorbent ratio),10b and
can be attributed to the large porosity as well as the distinct
hydrophilicity of the O and N components that help the water
Results and Discussions
Preparation and analysis of HOTT-HATN
HOTT-HATN was assembled using a high-yield, metal-free
aromatic nucleophilic substitution,9-10, 11 and simply involves
heating
the
two
monomers
(HOTT) and
2,3,6,7,10,11-
2,3,8,9,14,15-
hexahydroxytriphenylene
hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene (HATN-Cl6,
Fig. 1) under N2 at 170°C for five days using K2CO3 as the base
and DMA as the solvent. After purification via Soxhlet
extraction, a dark red brown solid was recovered in a high
yield (89%).
Results of CHN elemental analysis fits the composition
(C42H12O6N6)·(H2O)10, with C42H12O6N6 being that of the 2D net
of Fig. 1 (see ESI for details). Energy dispersive X-ray spectra
(EDX) reveals only a trace amount of Cl (< 0.2 atom %, Fig. S3),
phase better permeate the polymer matrix. Compared with
the thiol-laced PPF systems,10b,
10c
HOTT-HATN is easier to
prepare, and offers better stability, being less prone to
oxidation.
Fig. 2. N2 isotherm at 77 K of an activated sample of HOTT-HATN.
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Fig. 3. a) Adsorption curve of Pb(II) versus contact time with HOTT-HATN in aqueous solution. Inset: linear first-order kinetics
plot for the initial four points (the Pb levels are below detection limit starting from t = 5 min). b) A Pb(II) Langmuir sorption
isotherm for HOTT-HATN. Inset: linear expression fitted with the Langmuir model.
Correct interpretation of kinetics data in adsorption studies have CiV<<qe instead and the first order description instead
is of topical interests (e.g., see Chao’s recent review).16 The becomes more meaningful.
present PPF system, with its large porosity and well-defined
Second, the linearity between t/qt and t/qe is often
2
binding sites, appears fitting for the Langmuir model. The gratuitous, especially in fast kinetics where 1/(k2qe ) is of small,
ꢀ
ꢃ
ꢄ
kinetic data indeed fit the first order equation ln ꢁꢂ
negligible values, rendering t/qt ≈ t/qe. At longer time points,
2
ꢅꢆꢇꢈ ꢉ lnꢀꢁꢊꢃ (Fig. 3a inset), which can be rationalized using t/qe becomes ever greater than 1/(k2qe ), making the linearity
17
⁄
the Langmuir model ꢋꢁꢂ ꢋꢈ ꢄ ꢆꢌꢂꢁꢂ
,
where ꢁꢂ (mg·L-1) a triviality further devoid of physical significance; also at longer
refers to the remaining Pb in the solution, ꢌꢂ the number of times, the adsorption approaches equilibrium, and the reverse
unoccupied sites of the sorbent at time (min), and
(g-1min- process—the desorption—becomes significant, further
1) the rate constant—with ꢌꢂ approximated to be a constant. invalidating the one-way pseudo second order assumption.16a
ꢈ
ꢆ
Such an approximation is justified in the present condition, Indeed, with excess adsorbent (i.e., CiV<<qe), the qe value
in which excess adsorbent was used so that the sorption sites derived from the superficially linear plot between t/qt and t/qe
greatly outnumber the Pb ions, e.g., in the above test the is simply the total meal ions present, having nothing to do with
sorbent (20 mg) offers sites for 1.34 mg of Pb uptake, over 2.6 the number of available adsorption sites physically present.
times the Pb ions present (0.5 mg). In general, drinking water
To properly assess the sorbent kinetics in connection with
treatment usually involves heavy metal pollutants at very low the removal of trace heavy metal contaminants, we suggest
concentrations (e.g., sub-ppm), and the adsorbent is deployed that excess adsorbent be used in order to mimic the actual
in large excess to fully suppress the residual heavy metal deployment of the adsorbent, and to simplify the kinetics to
content. In other words, only a small fraction of the binding the first order regime. Also, data points should be collected at
sites of the adsorbent will be occupied, which further justifies the early stage so as to avoid the complication from
the first order assumption. The linear plot based on the first desorption. For example, with the distribution quotient Qd =
four data points (the Pb became undetectable by the 5th (Ci-Ct)V/Ctm < Kd/100, the desorption rate is no greater than
point) yields a ꢆꢇ of 1.86 min-1 (Fig. 3a inset), with the 1% that of adsorption, and can thus be omitted. As adsorption
corresponding half-life being 0.37 min.
isotherm is routinely measured, the adsorption capacity qe and
The kinetic data also fit the pseudo-second-order kinetic Kd derived thereby provide valuable guidance and cross-check
model10c, 18 ꢋꢌꢂ ꢋꢈ ꢄ ꢆꢍꢎꢀꢌꢏ ꢅ ꢌꢂꢃꢎ with the following linear for the kinetic studies. For benchmarking the kinetic
⁄
fit:
performance, the first order rate constant k1 can be divided by
the amount of sorbent
m (g). In the above case,
ꢈ
ꢌꢂ
1
ꢆꢎꢌꢏꢎ
ꢈ
ꢌꢏ
k1/m=1.86/0.020=93 min-1·g-1.
ꢄ
ꢉ
ꢀ1ꢃ
In the isotherm study, a series of 5.0 mL aqueous Pb(NO3)2
solutions were loaded into glass vials (each containing 5.0 mg
HOTT-HATN). The mixtures were shaken at 200 rpm for 4.5 h
at rt, then filtered by PTFE membrane. The filtrates were
tested via ICP-AES. The adsorbed amount of Pb at equilibrium
where ꢌꢂ (mg·g-1) is the amount of Pb adsorbed at time
ꢈ
(min), and ꢌꢏ (mg·g-1) is the Pb adsorbed at equilibrium, and ꢆꢎ
(g·mg-1min-1) is the adsorption rate constant (Fig. S13).
Such a fit, however, should be taken with caution.16a, 16c
Theoretically, the pseudo second order model works best
when the total number of the metal ions equals that of the
adsorption sites, i.e., CiV = qe, namely CtV = CiV- qt = qe-qt, so
that (qe-qt)2 = (CiV-qt)(qe-qt) = (1/V)(Ci-qt/V)(qe-qt) = (1/V)(Ct)(qe-
qt). When excess sorbent was used (as is often the case), we
(
ꢌ
ꢏ, mg·g-1) was calculated with the data in Table S3:
ꢀꢁꢊ ꢅ ꢁꢏꢃ ꢐ ꢑ
ꢌꢏ ꢄ
ꢀ2ꢃ
ꢒ
where ꢁꢊ and ꢁꢏ correspond to initial and equilibrium
concentration of Pb in water (mg·L-1),
stands for the solution
ꢑ
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volume (mL) and ꢒ the sorbent mass (g). The adsorption higher concentrations. Notably, the large majority of the low-
isotherm plots fit the Langmuir isotherm model (with a toxicity ions of Zn2+, Ca2+ and Na+ were not adsorbed, pointing
correlation coefficient above 0.998; see also Fig. 3b):
to further relevance for drinking water treatment. The uptake
of the toxic Cd2+ ions, however, was only modest, with 65% of
the Cd2+ ions found to persist in the solution in these
experimental conditions.
ꢁꢏ
ꢌꢏ
1
1
ꢄ ꢁꢏ ꢐ
ꢉ
ꢀ3ꢃ
ꢌꢓꢔꢕ ꢖꢗꢌꢓꢔꢕ
where a plot of Ce/qe to Ce (see Fig. 3b inset) yields the
sorption capacity ꢌꢓꢔꢕ (mg·g-1) as the reciprocal of the slope.
The ꢌꢓꢔꢕ value thus obtained (66.8 mg·g-1) is equivalent to
0.28 Pb(II) ion per HATN core, suggesting that a large fraction
of bipy donors remain unbonded under these conditions.
The strong affinity for Pb(II) ions is demonstrated by the
Knoevenagel condensation reactions
We now explore HOTT-HATN for green and sustainable uses in
chemical syntheses. Besides convenient product separation
(generally associated with heterogeneous catalysts), its metal-
free nature resolves the metal leaching and contamination
issue that besets especially electronic and pharmaceutical
industries.6b The hydrophilicity of HOTT-HATN (e.g., dispersible
in water), together with its oxo-strengthened pyridinyl donors,
also makes for efficient water-based heterogeneous
organocatalysis. The well-known Knoevenagel condensation
offers a useful test reaction, because it is amenable to base
catalysis, and it is an important C-C bond formation method
for fine chemicals and pharmaceuticals.21
distribution coefficient (ꢖꢘ), which is defined as:
ꢁꢊ ꢅ ꢁꢙ
ꢑ
ꢒ
ꢖꢘ ꢄ
ꢁꢙ
ꢐ
ꢀ4ꢃ
with Ci and Cf being the initial and final Pb concentrations, V
the volume of the solution (mL) and the amount of sorbent
ꢒ
(g). From the isotherm data (e.g., Table S3, Entry 4), ꢖꢘ was
estimated to be greater than 2×106 mL·g-1. The effective Pb
removal capability is consistent with the strong donor
character of the chelating pyridinyl groups. Specifically, such
chelating ability is greatly enhanced by the strongly electron-
releasing dioxin-like oxo groups built into the conjugated
backbone. To better demonstrate the essential role of the
nitrogen donors in Pb uptake, a sample of HOTT-HATN
polymer was treated with acetic acid to help mask the alkaline
pyridinyl sites; consequently, the Pb uptake capacity was
found to drop down to about 30 mg·g-1 (see Fig. S14), being
less than half that of the pristine sample. Such sensitivity of
the Pb uptake property to acid treatment indicates that the Pb
uptake is effected mostly through the alkaline N donor
functions.
Compared with other sorbent systems for Pb
removal
19the fast kinetics and high Kd value of HOTT-HATN
,
clearly rank among the best, even though the uptake capacity
is modest. The extraordinary Kd value, however, is especially
useful for removing Pb from drinking water (e.g., down to 15
ppb as is set by the US Environmental Protection Agency).20
Specifically, with Kd = 2×106, for one liter of water with Pb at
150 ppb (i.e., 10 times the legal limit), only 4.5 mg of the
HOTT-HATN polymer is needed to reduce the Pb content to 15
ppb. Further to the advantage of product economics, the
HOTT-HATN sorbent can be reactivated after stripping off the
trapped Pb by sequential washing with the household items of
HCl (e.g., 10%), ammonia (5%) and water (see SI for details,
e.g., Fig. S15).
The selectivity of heavy metal uptake was examined using a
solution containing Hg2+, Pb2+, Cu2+, Cd2+ and Zn2+ at 4.8-6.4
ppm (see Tables S4, S5), together with Ca2+ and Na+ at 20 and
53 ppm (to mimic the higher natural occurrence of these two
metals), respectively. As shown in Fig. S16, strongest
preferences were observed for Hg2+, Cu2+, with over 95%
removal achieved within one hour, while the adsorption for
Pb2+ continues to be effective, with about 90% removal
achieved in spite of the various competing ions present at
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CHCl3 added for substrates with poor water solubility). The
reaction vessel was then heated to 40 °C and shaken at 250
rpm for 2.5-3 hours, and the conversion rate determined via
1H NMR (see ESI). A benchmark reaction was first performed
with benzaldehyde, resulting in 100% conversion after 2.5
hours. Also examined are controls for solvents (chloroform
and acetonitrile) in the absence of water as well as a reaction
in the absence of HOTT-HATN (Table 1). Interestingly,
replacing water with pure CDCl3 dramatically slows down the
reaction, e.g., even after 12 hours, only 67% conversion was
observed, while straight acetonitrile resulted in (slightly
higher) 80% conversion in similar conditions. Also, the
absence of HOTT-HATN resulted in a much lower yield (i.e., cf
Table 1. Knoevenagel condensation of aldehydes with malononitrile catalyzed by HOTT-
HATNa
HOTT-HATN polymer
CN
+
NC
CN
H2O, 40
C
O
CN
R
R
shaker 250 rpm
Entry
1
Aldehyde
Reaction
Conversion (%)b
>99
time (h)
2.5
2c
3d
4e
5f
6
12
12
2.5
2.5
2.5
3
67
80
entries
1 and 4), verifying HOTT-HATN as a useful
organocatalyst.
62
Substituted benzaldehydes (4-fluoro, 2-bromo, 4-hydroxy
and 2-bromo-4,5-dimethoxy) were also tested to open the
scope of application. Each of these showed full conversion
within 3 hours. The bulkier substrate (3,5-bis(benzyloxy)-
>99
>99
>99
>99
>99
benzaldehyde, entry 10), however, showed
a lower
conversion of 82% even after 15 hours, presumably due to its
difficulty in penetrating the HOTT-HATN matrix. Finally, HOTT-
HATN was recovered, washed and reused throughout five
catalytic cycles of the model reaction. No significant decrease
in catalytic activity was observed therein, further establishing
HOTT-HATN as a green and reusable heterogeneous catalyst
with efficiency comparing favorably with various other
systems.22
7
8
3
9
3
10
15
82
Aerobic photooxidation of benzylamines
Provided with appropriate photocatalysts, light—especially
abundant sunlight—offers an attractive alternative energy
input (vis-à-vis heat) for driving chemical transformations. In
spite of various CMP solids studied as photocatalyts for
organic reactions, the field remains wide open.23 For example,
rigorously metal-free CMP photocatalysts are rare, and the
continuous flow reactors, well-suited to enable scale-up and
to overcome other operational challenges of photochemistry,
are curiously often left out in CMP photocatalysis studies.
The oxo and aza functions built into the fully conjugated
backbone of HOTT-HATN constitute distinct donor-acceptor
units that give rise to broad intense absorption in the visible
region (Fig. S7), indicating potential photocatalytic activities.
The following continuous-flow synthesis using HOTT-HATN as a
photocatalyst aims to combine a unique set of green-
chemistry merits. The efficient production of imines thus
achieved is also of practical interest in the synthesis of
nitrogen-rich biologically molecules.23-24
aReaction conditions: aldehyde (0.25 mmol), malononitrile (0.375 mmol),
H2O (1.0 mL), CHCl3 (0-0.2 mL), HOTT-HATN (5 mg), mechanical shaker (250
c
rpm), 40 °C. bConversion calculated via 1H NMR analysis. CDCl3 (no water
added), 50 °C by heating block, stirring. dAcetonitrile (no water added), 50
°C by heating block, stirring. eIn water without HOTT-HATN. fHOTT-HATN
polymer re-used in the 5th cycle.
The Knoevenagel condensations involved adding an aryl
aldehyde (0.25 mmol), malononitrile (0.375 mmol) and HOTT-
HATN (5.0 mg) to 1.0 mL of deionized water (with 0.2 mL of
For the photooxidation, a benzyl amine (0.5 mmol) and
HOTT-HATN (5.0 mg, ca 6.5 µmol of the HATT unit) were added
to 10 mL of acetonitrile. The suspension was then cycled
through a commercial photochemical flow reactor at rt
(Vapourtec Ltd., Fig. S17) in the presence of oxygen and visible
light (>400 nm, Cool White LED module, Fig. 4) until the amine
was consumed. The conversion rate was measured by 1H NMR
(see ESI).
Fig. 4. Schematic representation of the experimental set-up using the easy-
Photochem flow reactor from Vapourtec Ltd. with the spectrum for the Cool White
LED module (>400 nm).
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Table 2. Aerobic oxidative coupling of benzyl amines photocatalyzed by HOTT-HATNa
were not greatly significant. Specifically, benzyl amines with
electron-donating groups took slightly less time to reach full
conversion (5 hours), while those with strong electron-
withdrawing groups took longer (7 h). Recently, the
photooxidation of benzylamine has been described as
proceeding through either a photoredox or singlet oxygen
(1O2) mechanism.25 Through a photoredox mechanism, the
amine substrate transfers an electron to the triplet excited
state of HOTT-HATN. The photoreduced HOTT-HATN
subsequently interacts with O2 to form a superoxide radical
anion (O2˙-), and thus returning to the ground state. The amine
radical is then oxidized via O2˙-, where the intermediate amine
is able to react with the initial amine substrate to form the
desired product. Conversely, the production of 1O2 proceeds
via an energy transfer between O2 and the triplet excited state
of HOTT-HATN. The amine substrate is then oxidized by 1O2
and proceeds to the product in a similar manner as described
for the photoredox mechanism.
Entry
1
Amine
Reaction
time (h)
Conversion (%)b
6
6
6
6
4
6
5
5
6
6
7
>99
2
2c
3d
4e
5f
14
-
>99
92
To better understand how the reaction proceeds, the
ability for HOTT-HATN to be used for the production of 1O2 was
investigated through the conversion of α-terpinene to
ascaridole, as described in our previous work.26 After 2 h
irradiation with a Cool While LED Module (>400 nm), the
reaction showed a 62 % conversion to the ascaridole product
via 1H NMR spectroscopy, indicating 1O2 production under
experimental conditions similar to those presented for the
photooxidation experiments. Therefore, we can reasonably
conclude that the photooxidation of benzylamines is most
likely proceeding through a 1O2 mechanism.
6g
7
>99
>99
>99
>99
>99
8
9
10
11
Lastly, we investigated both the photostability and
productivity of HOTT-HATN. To test the material for
photostability, HOTT-HATN was added to acetonitrile and
irradiated for 30 hours in the presence of oxygen. The
irradiated suspension was then used for the model reaction
where 92% conversion was achieved within 6 h, establishing
that HOTT-HATN maintains high catalytic activity, and
therefore photostability, even after prolonged exposure to
high intensity light. FTIR analysis of the polymer before and
after 36 h of intense irradiation (Fig. S37) presented with
virtually no change, further indicating the stability of HOTT-
HATN. We also note that 1O2 is generated throughout the
entirety of the irradiation period, cementing HOTT-HATN as
not only photostable, but also inert under highly oxidative
conditions. Furthermore, with the same amounts of HOTT-
HATN (5.0 mg) and solvent, the benzylamine can be raised in
concentration (from 0.05) up to 0.08 mmol·mL-1 and still
aReaction conditions: benzyl amine (0.5 mmol), HOTT-HATN (5.0 mg),
acetonitrile (10 mL), 30 °C, visible cool white led module (>400 nm), in
flow (1 mL·min-1), O2 (1 mL·min-1). bConversion calculated via 1H NMR
spectroscopy. cReaction performed in the absence of light. dReaction
performed in the absence of HOTT-HATN. eReaction performed in the
absence of O2, under N2 atmosphere. fReaction performed using a 420 nm
LED module. gHOTT-HATN in acetonitrile irradiated for 30 h open to air
prior to addition of benzylamine.
As seen in Table 2, a series of benzylic amines were
examined. A screening was first performed with benzylamine,
resulting in full conversion to the imine product within 6 hours.
Control experiments, removing light, photocatalyst or oxygen,
were also performed to verify the role of each of these
components. Furthermore, heating (at 60°C) under the initial
reaction conditions did not significantly change the final
conversion of the reactions. The use of a narrow wavelength
light source (420 ± 2 nm) reduced the reaction time to 4 h.
However, due to the abundance of white light sources (as in
household lighting), we opted to continue with the Cool White
LED module.
demonstrate full conversion within
6
hours (higher
concentrations require longer reaction time), equivalent to a
TON above 120. Overall, the photocatalytic activity of HOTT-
HATN compares well with other photocatalysts (both metal-
rich and metal-free) for oxidative coupling of benzyl amines.23
Conclusions
Substituted benzyl amines (4-methoxy, 4-methyl, 4-fluoro,
4-chloro, and 4-trifluoromethyl) were also tested to probe
scope of substrates. While we note some variation in the time
required for full conversion, the differences in these times
In conclusion, an impressive array of green credentials has
been achieved from
a highly reusable, fused-aromatic
framework efficiently assembled via a transition-metal-free
6 | J. Name., 2012, 00, 1-3
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COMMUNICATION
5. F. Vilela, K. Zhang and M. Antonietti, Energy Environ. Sci., 2012,
5, 7819-7832.
substitution reaction between the building blocks of HOTT and
HATN-Cl6. In particular, the oxo and the aza sites built into the
conjugated polymer backbone provide a powerful electron
donor-acceptor couple that effects the photocatalysis of
aerobic amine oxidation using visible light when applied in a
commercial flow reactor. Moreover, the oxo units also
enhance the basicity of the pyridinyl aza units, so as to
effectively catalyze the Knoevenagel reaction in water, and aid
in the speedy removal of Pb from water to acceptable drinking
levels set by worldwide environmental organizations. This
latter capability is especially of note not only because of the
convenient recyclability of the polymer matrix (e.g., by
washing with HCl), but also because of the widespread
presence of lead as a pollutant in the environment.
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Acknowledgements
This work is supported by a GRF grant of Research Grants
Council of Hong Kong SAR (Project 11303414), the National
Natural Science Foundation of China (21471037) and
Guangdong Natural Science Funds for Distinguished Young
Scholars (15ZK0307). We acknowledge Vapourtec Ltd for their
technical support.
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A metal-free conjugated porous polymer offers a remarkable range of capabilities crucial for
green and sustainable technologies, including: removing 99.9% of lead from water within
minutes; catalyzing quantitative Knoevenagel reaction in water; effective donor-acceptor units
boosting photocatalytic activity and the implementation in a continuous flow reactor.