Guanine-Derived Porous Carbonaceous Materials: Towards C1N1

Article abstract of DOI:10.1002/cssc.202002274

Herein, the basic nature of noble covalent, sp2-conjugated materials prepared via direct condensation of guanine in the presence of an inorganic salt melt as structure directing agent was studied. At temperatures below 700 °C stable and more basic additio

Full text of DOI:10.1002/cssc.202002274

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Guanine Derived Porous Carbonaceous Materials: towards C1N1
Authors: Janina Kossmann, Tobias Heil, Markus Antonietti, and Nieves
López-Salas
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01/2020

10.1002/cssc.202002274
ChemSusChem
RESEARCH ARTICLE
Guanine Derived Porous Carbonaceous Materials: towards C1N1
Janina Kossmann[a], Dr. Tobias Heil[a], Prof. Dr. Dr. Markus Antonietti[a] and Nieves López-Salas[a]*
[a]
Colloid Chemistry Department
Max Planck Institute of Colloids and Interfaces
Am Mühlenberg 1, 14476, Potsdam
E-mail: nieves.lopezsalas@mpikg.mpg.de
Supporting information for this article is given via a link at the end of the document.
Abstract: Herein, we study the basic nature of noble covalent, sp2-
conjugated materials prepared via direct condensation of guanine in
the presence of an inorganic salt melt as structure directing agent. At
temperatures below 700 °C stable and more basic addition products
with at C:N ratio of 1 (C1N1 adducts) and with rather uniform micropore
sizes are formed. Carbonization at higher temperatures breaks the
structural motif, and N-doped carbons with 11 wt% and surface areas
of 1900 m2 g-1 are obtained. The capability for CO2 sorption and
catalytic activity of the materials depend of both their basicity and their
pore morphology. The optimization of the synthetic parameters lead
to very active (100% conversion) and highly selective (99% selectivity)
heterogeneous base catalysts, as exemplified with the model
Knoevenagel condensation of benzaldehyde with malononitrile. The
high stability upon oxidation of these covalent materials and their
basicity open new perspectives in heterogeneous organocatalysis.
system changes the collective electronic properties and
thereby also their acid/base strength.[8] Among the different
heteroatoms introduced on carbonaceous networks (e.g., S, N,
P or B), nitrogen is the one that has been most extensively
explored by far, due to its capability to facilitate CO2
adsorption.[9] Nitrogen doping is normally achieved either by
condensation of nitrogen rich organic precursors (e.g.
polyaniline, pDOPA) or by post-treatment of the covalent
material (e.g. impregnation of nitrogen rich precursors or heat
treatment in ammonia atmosphere), while the amount of
nitrogen seems to be limited in such thermal “shotgun
processes”.[10] Some previous works reported nitrogen
contents up to 36 wt% by condensation of nitrogen rich
compounds up to 500 °C.[9a, 11] However, upon increasing the
condensation temperature further, the nitrogen content
decreased to lower values (e.g. 10 wt% at 1000 °C). Such
species seem to be thermodynamically stable and justify the
notation “N-doped carbon”. Therefore, in order to prepare
better basic materials in the present context, it is mandatory
to control the process and appropriate condensation
chemistry at a given temperature.
Introduction
Producing “carbonized” materials has always been considered
an uncontrolled process. In 2010 Paraknowitsch et al.[1] and
Lee et al.[2] described independently and almost
simultaneously the production of carbons from direct
pyrolysis of ionic liquids with very special properties (i.e., high
temperature resistance under air atmosphere). Later, in 2015,
Sakaushi et al.[3] extended the principle to nucleobases,
specifically the direct condensation of adenosine. Common to
all these new carbons is that the original precursors used were
very stable compounds (i.e., that possess a HOMO level more
positive than 1.3 V). Thus, their carbonization led to even more
stable carbonaceous materials due to the bond formation
upon carbonization being limited to a number of even more
stable motives.[4] Such a hindered bond formation promotes
the formation of sp2-conjugated and layered materials, as
aromatic packing is energetically favorable. More
interestingly, such stable initial compounds present mostly a
high heteroatom content that is usually transferred by a large
extent to the final carbonaceous products. Materials fulfilling
Carbonaceous materials, or rather high temperature organic
condensation products, have been the materials of choice for
a myriad of applications due to their simple large-scale
synthesis and the versatility of their physico-chemical
properties.[12] Nowadays, gas adsorption and electrochemical
energy storage and conversion are among the most common
application fields. Interestingly, in 1855, J. Stenhouse already
probed that carbon had potential in the field of catalysis, but
[13]
it was not until the bloom of graphene related materials
that carbocatalysis earned well deserved attention.[14] We
have recently reviewed the origin of the catalytic activity of
carbon materials for those reactions, which otherwise are
commonly carried out with transition metals, with the aim to
highlight where, in our opinion, carbons have great potential
to outperform benchmark catalysts.[14-15] Among others, and
despite their great potential in chemical synthesis, basic
heterogeneous catalysts stand out due to their scarcity. Some
reactions like transesterifications, aldol and Knoevenagel
condensation reactions have already been reported to be
catalyzed by carbonaceous materials.[16] There, the reaction
between malononitrile and benzaldehyde has also been used
as a simple way to test the basicity of carbon nitride.[17]
[4]
these conditions were labeled as “noble carbons”.
However, the concept of noble carbons is rather new and even
the already targeted potential precursors such as nucleobases
(e.g. adenine,[3, 5] guanine,[6] cytosine or thymine), purines,
xanthine derivatives or ionic liquids [7] have been only roughly
explored.
The basicity of heterogeneous materials is classically tuned by
surface functionalities. There is however a second option, as
the introduction of heteroatoms in the carbon-π electronic
Nucleobases not only are ubiquitous, they are Nature´s choice
to store its most important information. Thus, it is to no
surprise that they are indeed very stable and as such excellent
candidates as precursors towards controlled noble
1
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10.1002/cssc.202002274
ChemSusChem
RESEARCH ARTICLE
carbonaceous products with high heteroatom contents. products are water, carbon dioxide, and ammonia (see Figure
Guanine is essentially an oxidized pentamer of HCN[18] and S1). As the acidity is vanishing in this step, we have to expect
contains equimolar amounts of nitrogen and carbon, and of
course it is also easier to work with guanine than with HCN.
a condensation of the primary amine with the amido group. In
that case, the polymer would be left with a rather strong
The molecule as such starts condensing after its melting secondary amine. Above 500 °C, more carbon dioxide, water,
temperature at 365 °C. As it is oxidized, mostly water and and ammonia are detached while the carbon-nitrogen
equimolar amounts of CO2 and ammonia are produced during network apparently rearranges and further condenses (see
condensation, while depolymerization into HCN is suppressed.
Herein, we report a more careful analysis of this condensation
reaction in terms of the composition of the as formed solid
state organic materials using salt melts as solvents and
structure directing agent with the aim to show how precoded
information in noble carbonaceous precursors lead to the
formation of adducts with similar properties and basic
Figure S1). We will see below that this step is combined with
the formation of a structural, porous network. Between 700 °C
and 800 °C, massive loss of more heavy molecules, presumably
dicyan and its hydrogen adducts, is found, with a final solid
state residue of ca. 10 wt%. In this process, we have treated
guanine within a 1:1 NaCl/ZnCl2 salt melt (herein called SZ)
that acts as a solvent and structure directing agentand keeps
character. Guanine was treated at different temperatures (by strongly interacting with the precursor) functionalities in
using NaCl/ZnCl2 (1:1) salt mixture. The products obtained
were characterized by a variety of local techniques, including
the final covalent condensate, thus also leading to a higher
condensation yields.
thermogravimetrical analysis, Fourier transformed infrared A set of bulk samples were then produced by heating guanine
spectroscopy, elemental chemical analysis, scanning and up to 500, 600, 700 or 800 °C at 1 °C/min and then keeping
transmission electron microscopy, N2 and CO2 adsorption and the temperature for 2 h. In order to understand the influence
desorption isotherms, CO2 and temperature programmed of the dilution on the final product properties, different salt
desorption.
We obtained three different classes of products: up to 500 °C, used. Samples are named hereafter as cG@t-SZx where t
a non-porous polyguanine was obtained, which transformed stands for the condensation temperature and x for the
into materials with C/N ratio of 1, i.e., C1N1 adducts, with guanine to salt melt ratio. Table 1 shows a summary of the
structural porosity that was stable until 700 °C. Further prepared samples and their measured composition (by both
melt ratios (i.e., 1:1, 1:6 or 1:10 guanine:salt melt) were also
heating to 800 °C resulted into depolymerization and energy disperse X-ray spectroscopy – EDX – and elemental
elimination of dicyan and a remaining highly porous N-doped chemical analysis – ECA). Samples prepared at 500 °C, 600 °C,
carbon. The resulting samples were then evaluated in CO2 and 700 °C show a C/N ratio of ca. 1 (similar to that of
adsorption and as a heterogeneous catalyst for a Knoevenagel
condensation reaction, both quantifying the basicity of the
materials. It turns out that all condensates up to 700 °C show
guanine), independently of the amount of salt used. On the
contrary, at 800 °C the C/N ratio raises substantially and by
increasing the salt content it changes from 3 to 7. Regarding
high basicity, which drops down to ordinary nitrogen basicity the samples yield (Table S1), the amount of condense material
for the final N-doped carbon.
obtained decreases with increasing temperature, while the
yield only weakly depends on the amount of salt. These
findings indicate the formation of an amorphous material with
at C/N ratio of 1 (C1N1 adduct) that is stable up to 700 °C and
is not altered by dilution in the molten salt solvent throughout
condensation. At 800 ºC the condensation product has lost its
Results and Discussion
Guanine is an appealing candidate for the production of basic
carbonaceous materials due to its high N/C ratio, the presence
of amidic functionalities and precoded potential n-carbene
sites. Guanine as such has under standard conditions 3
acidic/basic sites, pKa = 3.3 (amide), 9.2 (secondary), 12.3
(primary). Mass spectroscopy coupled to thermogravimetrical
analysis (TGA-MS) of guanine indicates that there is a first
mass loss ending at ca. 500 °C, where the major decomposition
Table 1. Summary of the composition data obtained by EDX, ECA, and ICP analyses and data extracted from physisorption analyses.
CO2
c
d
EDX / ECA (wt.%)
SSABET
VT
Pore diam. e
Sample
adsorption f
[mmol g-1]
C
N
O
Ha
Znb
2.1
C/N
1.1/
1.1
1.3/
1.1
1.1
7.4/
6.5
[m2g-1]
8
[cm3g-1]
-
[nm]
-
47.7/
38.0
51.7/
38.0
46.6
81.2/
72.0
43.0/
35.0
41.0/
34.0
42.4
11.0/
12.0
3.4/
14.9
7.2/
17.0
4.4
cG@500-SZ10
1.6
1.6
cG@600-SZ10
cG@700-SZ10
cG@800-SZ10
3.0
-
2.7
4.0
0.3
852
812
0.41 (0.42)
0.38 (0.43)
1.40 (1.42)
-
3.9
3.9
4.7
1.2 (1.3)
6.7/
10.8
1.0
1982
4.1 (3.6) - (1.7)
a Only by ECA, b ICP, c obtained by applying the BET method to N2 adsorption isotherms at 77K, d calculated by DFT, in brackets the result obtained at P/P0=0.95, e
2
pore diameter as obtained from QSDT method applied to N2 adsorption isotherms at 77K (in brackets the values obtained applying the same model to Ar adsorption
isotherms at 87K) and f amount of CO2 adsorbed at 273K at 800 torr.
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10.1002/cssc.202002274
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Figure 1. (A) FTIR, (B) TGA traces in synthetic air atmosphere, and (C) XRD pattern of samples prepared at different temperatures.
fixed composition and the yield becomes dependent on samples condensed at 500 °C, 600 °C, and 700 °C show three
dilution. The strong interaction between the salt melt medium
main peaks centered at ca. 1210 cm-1, 1360 cm-1 and 1550
and the reacting phase plays here a crucial role stabilizing the cm-1 which can be ascribed to C-N functionalities, and the
leaving functional groups. As a result, the carbonaceous yield typical D and G C-C bands. The deconvolution of cG@800-SZ10
increases at higher dilution rates. It is known that ZnCl2 can act show that, when the temperature is risen to 800ºC, the G band
as dehydrating and condensing catalyst when present in salt
intensity decreases and a fourth peak at 1495 cm-1 appears.
melts. To address the contribution of ZnCl2 to the formation of This peak is usually ascribed to a D’’ mode of carbon. The X-
C1N1, a sample was prepared using a ZnCl2 free salt melt (i.e., ray diffraction (XRD) pattern (Figure 1c) of samples cG@500-
a 1:1 g/g mix of LiCl and KCl) as template. The obtained SZ10, cG@600-SZ10 and cG@700-SZ10 are again very similar,
material exhibited virtually the same composition as and the slight decrease of the peak at 26.5° upon increasing
cG@600SZ (i.e., C/N ratio of 1). As can be seen in Table 1, EDX the condensation temperature might indicate an increase of
and ECA measured different values for the composition of the
amorphous content. The XRD pattern of cG@800-SZ10 show
samples. The composition found with EDX (measured in high virtually no graphitic diffraction peak, which might indicate
vacuum) looks fair for the bulk samples, while the high oxygen that at 800 ºC the presence of an amorphous phase is
content only found in ECA indicates a bias by massive
adsorption under standard conditions i.e. room temperature
dominant. (Figure 1 and Figure S3 XRD pattern of samples
with increasing salt melt (Figure S4) further corroborates this
and standard humidity. We will see below that this is mostly hypothesis. For instance, while increasing the presence of salt
water, which might spontaneously adsorb on the surface of melt seems to have no effect on the diffraction pattern of the
the carbons, i.e. the samples act as molecular sieves for water.
samples at 500 ºC, at 800 ºC a constant decrease of the
Moreover, the incomplete mass balanced of ECA corroborates diffraction features is observed.
the nobility of the samples, as the condensation products are
Thermogravimetrical analysis (TGA) in synthetic air of the
not fully combustible. We leave this observation to the reader samples showed how the resulting materials resist upon
as a warning as working with such polar materials always
comes with a perturbation of the measurements by adsorbed
environmental gases. Standard protocols to clean and degas
oxidation (Figure 1b and Figure S5), independently of their
chemical composition. All samples easily survive 400 °C, if not
500 °C in air, i.e. they are noble by character. Here it is
such porous materials otherwise applied to porous carbons important to highlight that all materials where degassed at
simply cannot be applied to C1N1 compounds, thus already 150 °C for 20 h under vacuum previous to the TGA
indicating that we deal with a completely new class of
materials.
Fourier transformed infrared spectra (FTIR) as well as Raman
spectra of the materials evidence the similarities of samples
experiments, as alerted by the ECA results. Without this
pretreatment, a much bigger mass loss at 100 °C due to water
adsorbed on the materials was detected by TGA-MS (Figure
S6). The harsh conditions necessary to remove the water in the
condensed at 500 °C, 600 °C, and 700 °C (see Figure 1a and samples indicate that the materials are able to bind polar hosts
Figure S2). FTIR spectra of the materials show a broad peak at strongly.
ca. 1200-1600 cm-1, which can be ascribed to the stretching X-ray photoelectron spectroscopy (XPS) helps elucidating
mode of CN heterocycles, and a broad band in the range whether carbon-nitrogen functionalities suffer dramatic
between 2800 and 3700 cm-1 that can be ascribed to water or changes upon increasing the temperature. Figure S7 shows
deformation and stretching modes of NH vibration. For the the deconvoluted N1s and C1s XPS spectra of cG@500-SZ10,
800 °C sample, the FTIR spectra do not show clear peaks cG@600-SZ10, and cG@800-SZ10. For all samples, the C1s
anymore, further highlighting the different nature of this signal is the result of adding up signals from C=C bonds (284.7
sample. Raman spectra of all the samples show very broad eV) and C-N bonds (286.1 eV) and a third deconvolution peak
peaks, indicating that, if any, the crystalline sites of the
samples are very small. The deconvolution of the spectra of
centered at 286.5 eV ascribed to C=O bonds (that can come
from both the material itself and adventitious CO2). When the
3
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We observe the transit from a total absence of any
microporous texture at 500 ºC to well homogeneous porous
materials. EDX elemental mapping of the samples shows that
nitrogen is homogenously distributed in the materials at every
temperature tested (see Figure S8). All this information
suggest that the primary condensate formed at 500 ºC slowly
rearranges into a more defined porous carbonaceous material
up to 700 ºC. Then, at 800 ºC, we obviously leave the
thermodynamic stability range of C1N1 and the former
material depolymerizes and leaves a nitrogen doped carbon
behind (as observed by XPS). This occurs under some slight
shrinking of the primary spheres and the development of a
typical carbon micropore structure, as reflected in TEM. Figure
S9 displays further micrographs obtained by SEM of samples
prepared with increasing guanine to salt melt ratio at both 500
°C (i.e., cG@500-SZ1, cG@500-SZ6, and cG@500-SZ10) and
800 °C (i.e., cG@800-SZ1, cG@800-SZ6, and cG@800-SZ10).
The images reveal how in both sets of samples the salt melt is
able to induce the formation of a carbon porous network and
follow similar trends upon dilution. N2 and CO2
adsorption/desorption isotherms help to understand the pore
morphology and to assess the basic nature of the samples.
When keeping a 1:10 guanine to salt melt ratio constant and
rising the condensation temperature from 500 to 800 °C,
samples above 500 °C start developing significant micropore
volumes and thus, large surface areas, as predicted by TEM
(see N2 adsorption/desorption isotherms in Figure 3a). As can
be seen, sample cG@500-SZ10 shows virtually no surface
area. Remarkably, both cG@600-SZ10 and cG@700-SZ10
exhibit a virtually equal type I N2 adsorption-desorption
isotherm. On the other hand, the cG@800-SZ10 isotherm
indicates that the sample not only presents a larger volume of
micropores, but also shows a sharp slope at intermediate
relative pressures. This might indicate that, while the
amorphous content increases, the occurrence of unstacking of
graphitic layers happens too. The formation of mesopores
would then be the result of the combination of both the
rupture of C1N1 due to the lack of stability at that temperature
and the larger evaporation of Zn ions from the salt melt used
as solvent and structure directing agent. The boiling point of
Figure 2. SEM (upper row) and TEM (lower row) micrographs of guanine condensed at
different temperatures with guanine to SZ ratio 1:10.
temperature rises to 800ºC the ratio between C=C and C-N
bonds increases, which is in good agreement with the
elemental composition. N1s signal deconvolution shows four
types of nitrogen functionalities: N-pyridinic (398.2 eV), N-
pyrrolic (399.7 eV), and a very electron poor N-quaternary in
plane (a broad band 405 eV). The ratio of the signals changes
when the temperature increases to 800 °C, with an increase of
electron poor nitrogen signals. The different intensities follow
the partial destruction of the C1N1 adduct. To summarize the
XPS results it can be seen that increasing the condensation
temperature shifts the C1s signal to lower binding energies
and N1s to higher ones. As basicity of nitrogen relies on rather
electron rich nitrogen atoms and is absent in N+-species, we
expect larger parts of basicity to be lost in the final
depolymerization step. Figure 2 compares scanning electron
microscopy (SEM) and transmission electron microscopy
images (TEM) of samples prepared at different temperatures
using a 1:10 guanine to salt melt ratio. Micrographs of all
series indicate the formation of a xerogel-like network
structure build up by primary particles with a diameter of 300
nm – 1000 nm. We assume that these particles precipitate
from the original homogenous salt solution once the
condensates reach a certain molecular weight in a spinodal
process, as it is typical for polymer colloids.[19] SEM shows how
all samples exhibit similar structures, whereas the TEM images
shows how within these primary spheres porosity develops.
Figure 3. (A) N2 adsorption/desorption isotherms at 77 K and (B) CO2 adsorption/desorption isotherms at 273 K of samples condensed at different temperatures using a
1:10 guanine to salt melt ratio and (C) water vapour adsorption/desorption isotherms of cG@700-SZ10 and cG@800-SZ10.
4
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10.1002/cssc.202002274
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RESEARCH ARTICLE
pure ZnCl2 and NaCl are 732 °C and 1461 °C, respectively. 600 °C and 700 ºC exhibit the same type of specific structural
However, as revealed by the TGA performed on the reacting sites plus the structural micropores that makes them
mixture, the decrease in mass (i.e., increase vapour pressure accessible to CO2 (i.e., not hidden anymore). Thus, they reach
of the metal halides) is already significant at lower
an adsorption capacity of up to 4 mmol g-1 of CO2. Where the
temperatures as seen in Figure S10 which further proof that shape of the isotherm indicated strong binding at already
the decomposition of C1N1 foster the formation of larger rather low pressures. The comparison with the internal
surface areas.[20] These changes provide cG@800-SZ10 with a reference of an N-doped carbon is indeed most rewarding. In
1982 m2g-1 specific surface area (SSA), whereas cG@600-SZ10 spite of the fact that the micropore volume observed by
and cG@700-SZ10 present a SSA of ca. 852 m2g-1. Such nitrogen physisorption for sample cG@800-SZ10 is much
sorption isotherms are similar to COFs and MOFs, but rather
untypical for ordinary activated carbons. For instance, the
explosion of surface area was already observed for the
larger (i.e., almost double), CO2 adsorption on this sample is
remarkably weaker and exceeds the values of cG@600-SZ10
and cG@700-SZ10 only at higher pressures and even then only
transition of density optimized CTFs to popped-up, thermally by ca. 0.5 mmol g-1. This means that the binding of CO2 in C1N1
delaminated CTFs.[21] Figure S11a and b give further sorption and N-doped carbons are fundamentally different, with a
data on the effect of temperature and salt amount. The figures much higher binding strength in C1N1. The 800 °C sample again
show the N2 adsorption/desorption isotherms of samples behaves similar to other highly N- doped porous carbons,
prepared at 500 °C and 800 °C, respectively, when increasing where CO2 uptake is mainly due a large micropore volume (i.e.,
the amount of salt melt in the synthesis. Remarkably, all
physical CO2 sorption). Figure S10c and d presents the CO2
isotherms at 500 °C show virtually no surface area while at 800 adsorption isotherms of samples prepared at 500 ºC (i.e.,
°C a remarkable increase on the final specific surface area
occurs with increasing amount of salt melt, again confirming
cG@500-SZ1, cG@500-SZ6, and cG@500-SZ10) and 800 °C
(i.e., cG@800-SZ1, cG@800-SZ6, and cG@800-SZ10) with
the nanoscopic “popcorn” effect. While the composition did different salt melt ratios. Samples made at 800 ºC show the
not change at 500 °C, the samples 800 °C show a change from same CO2 sorption capability independently of the differences
1:1 to 1:6 and then the composition is maintained though not on their surface area and apparent micropore volumes, as
anymore that C1N1 in any of the samples. This further proof determined by their N2 isotherms. Water vapour
that at this temperature the surface area development is the physisorption
(Figure 3C) reinforce the strong water
result of a the combination of ZnCl2 vapour pressure and adsorption assumed by ECA and TGA. cG@800-SZ10 shows a
rupture of C1N1 (Table S2). These results characterize the remarkable high water uptake of around 1500 cm3g-1 at high
three different types of materials throughout temperature
increase. At 500 ºC a solid with no accessible surface area
formed of precipitated guanine derived oligomers is obtained.
Though it has precipitated out of the salt melt, scaffolding has
not yet taken place and thus no porosity is observed. The
pressure, proving the highly polar character of the material.
Compared to cg@800-SZ10, the water uptake of cG@700-
SZ10 seems not as remarkable, but compared to other
carbonaceous materials, an uptake of ca. 420 cm3g-1 at high
pressures is still under the higher values reported up to
materials obtained at 600 °C and 700 °C are stable C1N1 porous date.[22] Moreover, the incomplete desorption of cG@700-
samples reminiscent to CTFs. As previous results evidenced, at SZ10 proves the strong binding of water to the polar surface
800 °C C1N1 is not stable anymore, and a nitrogen-doped as predicted by elemental analysis.
carbon with super high surface area is obtained due to sample
destacking and loss of nitrogen at high salt melt dilution. Ar measurements displayed in Figure 4a further corroborate the
adsorption-desorption isotherms at 87 K of cG@700-SZ10 and
cG@800-SZ10 were performed to further understand the
microporous nature of the samples. As can be seen in Figure
S12, both samples exhibit narrow distributions around 1.3 nm
and 1.7 nm respectively.
CO2 temperature programmed desorption (CO2 TPD)
CO2 isotherms not only further describe the pore morphology
of the samples, but also add information on possible acid-base
interactions. For instance, having a close look at Figure 3b, one
can see how CO2 adsorption isotherms of the samples at
different condensation temperatures change with structural
evolution. All the samples above 500 °C are able to adsorb
rather similar volumes of CO2, but curve shapes indicate how
different the CO2 adsorption sited in fact are. Samples at 500
Figure 4. (A) CO2 TPD traces obtained using cG@500-SZ10, cG@700-SZ10, and cG@800-
ºC show a remarkably high CO2 sorption despite their virtually
SZ10. Green area represents CO2 physisorbed in micropores, blue area represents CO2
bind to highly nitrogen doped pores, and yellow area is ascribed to CO2 bonded to
null N2 adsorption, while strong hysteresis points to the
kinetical hindrance throughout the process. C1N1 oligomeric
precipitates obtained at 500 ºC are able to uptake ca. 1.5
mmol g-1 of CO2 through very specific sites that are yet not
accessible (i.e., hidden) for nitrogen. C1N1 samples obtained at
structural C1N1 sites. (B) Conversion vs. time as catalyst for the Knoevenagel
condensation reaction of benzaldehyde with malononitrile. Solid lines stands for
materials used after degassing at 150 °C for 20h and dashed lines stands for materials
used without previous degassing.
5
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10.1002/cssc.202002274
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RESEARCH ARTICLE
different interaction of the three types of samples with CO2
(i.e., cG@500-SZ10, cG@700-SZ10, and cG@800-SZ10). Here,
it needs to be stated that the samples are loaded at ambient
temperatures and 1 bar of CO2, i.e. conditions where most
activated carbons already only partly bind CO2 at their surface.
The most intense signal is found for sample cG@500-SZ10. The
TPD trace shows two CO2 desorption broad peaks centered at
200 °C and 330 °C and no peaks up to 100 ºC, temperatures up
to which physisorbed CO2 would de-bound. This indicates that
CO2 binds to the material through two types of sites that
possess very high binding energies, well beyond physisorption.
Here, it is important to remember that the sample exhibits an
extremely low BET surface area, but still a surprisingly high CO2
binding capability of 1.5 mmol g-1.
Sample cG@700-SZ10 shows a similar CO2 TPD signal to that
of cG@500-SZ10, but a shoulder develops at temperatures
ranging from 150 to 200 °C, which we attribute to the
coexistence of some larger micropores. This sample has a
significantly larger SSA and a significant volume of micropores
which goes along with the capability of the sample to adsorb
up to 4 mmol g-1 of CO2 at room temperature. However, the
TPD shape indicates that the strong structural binding sites
remain until 700 ºC. In the TPD trace of cG@800-SZ10 (with
even larger SSA), a new peak at ca. 130 ºC appears (typically
ascribed to physisorption of CO2 on a noble carbon[23]) while
the peaks at higher temperatures are significantly lower in
intensity. This strongly indicates that at 800 °C only some of
the strong binding sites remain, but that most of the CO2
adsorption is either invisible in TPD, or remarkably weaker.
Nevertheless, even this sample still shows a very good CO2
binding capability, especially when compared to standard
activated carbons.
To analyse the basic character of cG@500-SZ10, cG@700-
SZ10, and cG@800-SZ10 as heterogeneous catalysts, the
Knoevenagel condensation reaction of benzaldehyde (1) with
malononitrile (2) was selected as a model reaction (see
Figure 5). The values of conversion of benzaldehyde (1) and
malononitrile (2) to 2-benzylidenemalononitrile (3) using the
different materials as catalysts are shown in Figure 4b and
Table 2. Samples were degassed at 150 °C for more than 20 h
under high vacuum previous to each experiment to ensure
that the active sites of the materials are fully available for the
reactants, as the materials spontaneously bind water and CO2
at ambient conditions. After this de-binding step, all catalysts
were more active and show 99 % selectivity towards (3). The
sample condensed at 700 °C shows the highest conversion of
99 % after 24h with 68% after 2h. cG@800-SZ10 also reaches
67% conversion after 2h. However, within the next 20h the
conversion reaches up to only 78 %. cG@500-SZ10, which
showed the most intense CO2 TPD trace, exhibit slower
kinetics. After 2h conversion reaches values under 10 % and
within the next 20h it slowly increases up to 50 %. The lower
activity of cG@500-SZ10 is ascribed to its almost non-existing
SSA, which makes the active sites unreachable for the
substrates, much larger than CO2. Additional different
substrates where tested to investigate the influence of
substitution of the aldehyde to the reactivity using cG@700-
Figure 5. Knoevenagel reaction followed in this paper.
SZ10 as catalyst. Substitution of benzaldehyde with chloride or
methyl groups in position 4 lower the conversion to 85 % and
68 %, respectively, which can be ascribed to the increasing size
of the substrate. Using furfural instead of benzaldehyde
further reduces the conversion to 46 %, which can be
explained by the change in the polarity induced by the furan
ring.
The majority of heterogeneous base catalysts in the literature
are pre-treated with strong bases to activate the base
functionalities (to de-bind potentially pre-bounded acids),
while simple degassing works for guanine derived porous
condensates (see Table S2).[17, 24] The results indicates that the
design of basic catalysts strongly depend on both their
composition and pore morphology, balancing the effects of
activation, activity and accessibility. Precoding information
using noble organic precursor is possible and offers great
possibilities, since the final catalyst not only could be reused
by simple heat treatments, but their high thermal and
oxidation stability allow their potential use in high
temperature catalytic reactions that are currently not
accessible by other covalent materials.
Conclusion
A series of noble covalent materials were prepared using
guanine as precursor and salt melts as solvent and structure
directing agent. Three different types of materials with very
different properties were obtained by increasing the thermal
condensation temperature. At 500 ºC oligomers with C/N ratio
of 1 (C1N1) precipitate out of the salt melted phase. By raising
the temperature up to 700 °C C1N1 adducts with well define
microporosity are formed. Chemical analysis of the samples
indicate that dilution in salt melts does not affect C1N1 at
temperatures below 700 ºC. However, at 800 °C, stability of
the adducts is lost, and popped up, high surface area, noble,
N-doped carbonaceous materials with a C7N composition are
obtained. Sorption experiments indicate that the structural
C1N1 pores bind CO2 with a strength previously not known
from covalent organic materials. The basicity of these
structural pores was proven by using the materials as
organocatalysts, and their performance was evaluated for the
Knoevenagel reaction between malononitrile and
benzaldehyde. Conversions and selectivities exceeding 99 %
could be reached, which excels above previously reported
catalysts that needed activation treatments to de-bind acidic
species. The samples stability and tunability opens the way to
utilize these new covalent materials in highly oxidative or high
temperature catalytic reactions that are currently not covered
by benchmark solid organic catalysts.
6
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10.1002/cssc.202002274
ChemSusChem
RESEARCH ARTICLE
(XPS) measurements were performed on a Thermo Fisher Scientific
Escalab 250 Xi. Temperature-programmed desorption of carbon dioxide
(CO2-TPD) was measured using a Micromeritics Autochem II 2920
analyzer. The sample (0.1 g) in a quartz microreactor was pretreated with
He (40cm3min-1) at 423 K for 60 min, cooled down to 323 K and the CO2
chemisorption were taken place in 20 consecutive pulses (25 vol% CO2/He
mixture, 1 cm3STP per pulse). After purging with He (10cm3STPmin-1) for
90 min, desorption was monitored until 673 K with a heating rate of 10
Kmin-1 and a He flow of 25 cm3STPmin-1.[25]
Experimental Section
Materials: Guanine 99% (G), sodium chloride (99.5%), zinc chloride,
acetonitrile, and benzaldehyde were purchased from Merck. Malonitrile
were purchased from Acros Organics and all were used as received.
Synthesis: The salt mixture was fresh prepared prior to each synthesis by
grinding NaCl and ZnCl2 in a 1:1 ratio (g/g) with a pestle and a mortar
(melting point: 325ºC). Covalent networks are prepared by simply mixing
guanine (1g) with different amounts of the salt mixture (i.e., 1g, 6g or 10g)
(see Table S1). All together were put into a ceramic crucible with a ceramic
cap and condensed under N2 atmosphere for 2 h at different temperatures
ranging from 500 °C to 800 °C using a 1 °C min-1 heating ramp. After
cooling down the samples were washed in 300 ml 1 M HCl three times and
dried at 70 °°C for 3 h.
Acknowledgements
We want to gratefully thank Heike Runge, Rona Pitschke and
Bolortuya Badamdorj for the microscopy work they
contributed to this investigation, Antje Völkel for measuring
TGA and ECA and the Max Planck Society is gratefully
acknowledged for financial support.
Catalytic test: Catalytic activity of the as prepared materials was tested
for the Knoevenagel condensation reaction of benzaldehyde with
malononitrile. In a typical reaction, 50 mg of catalyst were added to 10 ml
of acetonitrile solution containing 1 mmol of aldehyde and 2 mmol of
malononitrile (or ethylcyanacetate). The reaction mixtures were stirred at
70 °C for different times in an oil bath. Samples were evaluated using 1H-
NMR and GC-MS analysis. The product was identified by using an offline
gas chromatograph with an HP-5MS column (inner diameter=0.25mm,
length=30m, film=0.25µm) with a mass spectrometer (Agilent GC 6890,
Agilent MSD 5975). The percentage conversion were calculated by 1H-
NMR, recorded on Agilent 400 MHz.
Keywords: Basic solid material • Carbon dioxide fixation • CN •
Heterogeneous catalysis • Noble carbons
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RESEARCH ARTICLE
Entry for the Table of Contents
Heat treatment of guanine lead to a new material with a CN ratio of 1.1 (i.e. C1N1). The material shows structural micropores
(i.e. causing a strong CO2 binding), surface area (i.e. making them a promising candidate for heterogeneous catalysis) and stability
up to 700 ºC. At higher temperature, the structure popped up into highly nitrogen-doped carbons with much larger surface area.
This simple direct synthesis results in very promising new materials for several applications like CO2 capture and heterogeneous
catalysis.
9
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