Effect of solvent polarity in formation of perfectly ordered CMK-3 and CMK-5 carbon replicas by precipitation polycondensation of furfuryl alcohol

Microporous and Mesoporous Materials 329 (2022) 111542

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Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso

Effect of solvent polarity in formation of perfectly ordered CMK-3 and
CMK-5 carbon replicas by precipitation polycondensation of
furfuryl alcohol
´ ski c, Mariusz Wądrzyk a, b, Marek Lewandowski a, b, Piotr Łątka c,
Rafał Janus a, b, *, Piotr Natkan
Piotr Ku´strowski c
a

AGH University of Science and Technology, Faculty of Energy and Fuels, Al. A. Mickiewicza 30, 30-059, Krak´
ow, Poland
AGH University of Science and Technology, AGH Centre of Energy, Ul. Czarnowiejska 36, 30-054, Krak´
ow, Poland
c
Jagiellonian University, Faculty of Chemistry, Ul. Gronostajowa 2, 30-387, Krak´
ow, Poland
b

A R T I C L E I N F O

A B S T R A C T

Keywords:
CMK-3
CMK-5

Carbon replica
Poly(furfuryl alcohol)
Nanocasting
SBA-15

Two twin series of carbon replicas were synthesized by the acid-catalyzed precipitation polycondensation of
various amounts of furfuryl alcohol in SBA-15 suspensions using water and toluene as reaction media. The
textural and structural parameters, as well as the morphology of the polymer/silica carbonizates and corre­
sponding replicas, were investigated comprehensively. It was found that the polarity of the reaction medium
plays an essential role in the scenario of the deposition of poly(furfuryl alcohol) (PFA) onto the surface of the
silica matrix. Namely, the water-based environment results in propagating PFA chains radially from the pore
centres to the wall thereof, while in the case of toluene its growth progresses in the reverse direction. The
spectroscopic studies disclosed that this is due to the competitive adsorption of monomer and solvent on the
superficial silica silanol groups. In the case of the water-furfuryl alcohol system, H2O is adsorbed preferentially,
hindering the formation of a homogenous polymer layer, thus precluding the formation of a hollow-type replica.
Contrarily, for the toluene-furfuryl alcohol mixture, the monomer adsorption is favored. Furthermore, the
forming polymer anchors to the silica surface covalently and clads it evenly, therefore facilitating the formation
of a high-quality CMK-5 structure.

1. Introduction

esterification and transesterification, oxidative degradation) [10,
12–18], adsorptive hydrogen storage [9], purification (e.g. removal of
volatile organic compounds, NOx, and sulfur-containing compounds, as
well as CO2 capturing) [19–23], electrochemistry (as electrical double
layer (super)capacitors) [6,24–26], and medical purposes (mainly as
intracorporeal drug delivery carriers) [7,8,27–29]. Moreover, carbon
replicas are excellent model materials for a theoretical study of diffusion
and adsorption phenomena in porous solids [30–33], as well as XRD
patterns simulation/prediction [34]. Another interesting application

involves their use in the synthesis of mesoporous inorganic materials
(commonly metal oxides) featuring the structure of original silica
matrices (secondary replication of carbon structures) [35,36]. Attempts
were also made to synthesize metal oxides exactly imitating the struc­
tures of replicas [37]. Furthermore, it is well-documented that carbon
replication may be an ingenious tool for investigation of structures of
porous materials [1,38–42,47]. Recently, we reported on the elucidation

Ordered Mesoporous Carbons (OMCs), also called carbon replicas,
pose a class of nanoporous materials offering unique structural and
surface beneficial properties. They show such remarkable properties as a
long-range mesoscopic ordering, excellent homogeneity of pore shape
and size, highly developed specific surface area (up to ca. 2500 m2 g− 1),
and large total pore volume (even 2.5 cm3 g− 1) [1–4]. However, the
most desirable feature of OMCs is the opportunity of precise control of
their structure at the synthesis stage and ease of surface modification
[5–11]. With this, it is not surprising that in the last two decades carbon
replicas have attracted extensive interest in the scientific community,
especially for these purposes in which a well-defined porosity with a
long-range ordering is required. The favorable properties of OMCs
resulted in their successful applications as functional materials in a va­
riety of fields, including catalysis (e.g. hydrocarbons dehydrogenation,

* Corresponding author. AGH University of Science and Technology, Faculty of Energy and Fuels, Al. A. Mickiewicza 30, 30-059, Krak´
ow, Poland.
E-mail address: (R. Janus).
/>Received 20 September 2021; Received in revised form 26 October 2021; Accepted 29 October 2021
Available online 1 November 2021
1387-1811/© 2021 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license ( />

R. Janus et al.

Microporous and Mesoporous Materials 329 (2022) 111542

of the mechanism of pseudomorphic transformation (PT) of porous sil­
icas by non-direct investigation of the daughter carbon structures of the
SBA-15 upon partial PT into MCM-41 [43].
The pioneering synthesis of carbon replicas has been published in
1999 by the group of researchers from the Korea Advanced Institute of
Science and Technology (KAIST) [2]. The proposed synthetic route al­
lows the preparation of negative carbon structures (inverse replicas) cast
from porous silica materials (matrices) based on a so-called hard tem­
plating strategy. It consists in filling the pore system (either partial or
complete) of a mineral matrix with a proper carbon precursor followed
by carbonization of the composite and removal of the inorganic template
by etching with alkali or hydrofluoric acid. The first replica, called
CMK-1 (Carbon Mesostructured by KAIST) was synthesized by impreg­
nation of MCM-48 silica with an acidified sucrose solution as a carbon
source [2]. Inspired by Ryoo, other researchers put efforts to synthesize
a family of replicas employing silicas with different pore system ar­
rangements and a variety of carbon precursors used in various amounts.
The ultimate solids featured symmetry elements identical to the
matrices, although they were exact structural negatives thereof.
Furthermore, the partial pore filling of the silica matrix with a carbon
precursor may lead (but needs not) to the formation of open-work hol­
low-type frameworks, whereas total filling results in obtaining rod-type
replicas. The materials were marked with the common acronym CMK-n,
where n = 1–9 and differs depending on the matrix used and refers to the
ultimate carbon framework type [3].
The efficiency of carbon precursor deposition in the pores of silica

plays a crucial role in the quality of the resulting final replica (i.e. the
fidelity of matrix structure replication). Besides the aforementioned
impregnation, early methods of carbon precursor incorporation
included chemical vapor deposition (CVD). However, this approach
requires the use of an advanced apparatus and is time- and energyconsuming. Moreover, prior to the deposition of carbon precursor, the
matrix surface has to be properly modified (generation of active centres
catalyzing the polymerization of carbon precursor) [44–46]. Obviously,
such a sophisticated synthesis path precludes the possibility of utilizing
carbon replicas on a technical scale. Therefore, the reported application
tests, although gave very promising results, did not pass beyond the
laboratory scale, and attempts to synthesize high-quality hollow-type
structures (in fact, more challenging than the rod-type ones) have been
scarcely reported [3,18,47,48].
In our former study, we put efforts to develop a simplified route for
the synthesis of carbon replicas [12,49]. The novelty of our approach
consisted on employing the precipitation polymerization of carbon
precursor’s monomer onto the silica matrix walls in liquid media. Based
on this strategy, we successfully synthesized the CMK-3 replica by
nanocasting of the SBA-15 silica by the acid-catalyzed polycondensation
of furfuryl alcohol in an aqueous suspension of the rigid template. This
procedure led to the complete filling of the channel system of SiO2 and
allowed to shorten the synthesis time while using a green reaction me­
dium. Moreover, we managed to eliminate the step of preliminary
modification of silica.
These encouraging findings gave rise to undertaking attempts to
employ the same procedure to obtain the corresponding hollow-type
CMK-5 replica. Unexpectedly, the intended material has finally not been
obtained, although another interesting structure with bimodal meso­
porosity was created (the so-called pseudo-CMK-3) [15,50]. It was found
that the chemical nature of the medium used for the decoration of silica

with a polymer governs the manner of the carbon source deposition (the
homogeneous coating of the silica walls with polymer performed in the
water environment is not feasible). Furthermore, we hypothesized that
the solvent’s polarity and its possible interaction with the superficial
SBA-15 silanols may affect the mechanism of PFA deposition (e.g. due to
the competitive solvent-monomer adsorption hindering the homoge­
neous distribution of carbon precursor). This may influence (either
deteriorate or improve) the structural quality of the ultimate OMC.
Noteworthy, since our first report on PFA deposition in an aqueous

medium [12], there is a lack of research on the use of other media in the
literature. We have found this issue worth investigating as it is plausible
that the deposition of the carbon precursor in liquid media is more ho­
mogeneous than that of impregnation, being the most common pro­
cedure. In fact, the impregnation may be influenced by the local
fluctuations in the monomer concentration caused by the evaporation of
the solvent. Contrarily, the polymer precipitation in liquid media is a
self-regulating process driven by the affinity of the monomer to the
silica’s surface.
In this work we elucidate the role of the polarity of the medium used
for the precipitation of poly(furfuryl alcohol) onto SBA-15 silica matrix
walls on the mechanism of its deposition. This was feasible by the
investigation on textural and structural characteristics (N2 adsorption
and low-angle XRD, respectively), morphology (TEM), and spectro­
scopic study (FT-IR and XPS), which were carried out for two twin series
of replicas synthesized in water and toluene. It was found that using
polar solvent results in propagating polymer chains radially from the
bulk monomer solution to the silica pore wall, while in the case of
nonpolar medium their growth progresses in the reverse direction. As a
result, the polar medium precludes the formation of a hollow-type

replica, whereas the nonpolar solvent facilitates the formation of an
excellent CMK-5 structure. This finding may be a cornerstone to the
development of a simple and versatile method for the synthesis of other
carbon replicas.
2. Experimental section
2.1. Synthesis
All chemicals were commercially available and used without further
purification. Tetraethyl orthosilicate (TEOS, 98.0%) was purchased
from Acros Organics, poly(ethylene oxide)-block-poly(propylene oxide)block-poly(ethylene oxide) triblock copolymer (Pluronic P123), furfuryl
alcohol (FA, 98%), hydrofluoric acid (40–45%), potassium bromide
(≥99.0%), and isopropanol (≥99.5%) were supplied by Sigma-Aldrich,
whereas hydrochloric acid (35–38%, pure p.a.), tartaric acid (TA, pure
p.a.), toluene (pure p.a.), and sodium sulphate anhydrous (99.0%) were
purchased from Avantor Performance Materials Poland.
2.1.1. SBA-15
SBA-15 silica matrix was synthesized under acidic conditions at a
molar gel composition of 1.00 TEOS: 0.02 Pluronic P123: 2.94 HCl:
116.46 H2O according to the procedure reported elsewhere after fivefold
scale enlargement [12]. In brief, an amount of 85.00 g of TEOS (cooled
to 4 ◦ C) was slowly instilled (2 drops s− 1) and hydrolyzed at 35 ◦ C for 22
h in a mixture containing 40.00 g of Pluronic P123 dissolved before in
300.00 g of distilled water mixed with 600.00 g of 2 M HCl in a 2000 cm3
round-bottom flask placed in a silicone oil bath and equipped with a
reflux condenser. Subsequently, the milky reaction mixture was trans­
ferred to a laboratory dryer and kept statically at 100 ◦ C for 72 h (pre­
cipitate aging step). Then, the white product was recovered by filtration,
washed with 500 cm3 of distilled water, and dried at 60 ◦ C for 48 h.
Finally, the structure-directing agent was removed by calcination of the
silica/P123 composite in a muffle furnace under an air atmosphere at
550 ◦ C for 10 h at a heating rate of β = 1 ◦ C min− 1. The ultimate material

was marked as SBA-15. A small portion of as-made SBA-15 was calcined
using the identical thermal regime as for carbonization (850 ◦ C for 4 h, β
= 1 ◦ C min− 1). This sample was labelled as SBA-15@850.
2.1.2. Carbon replicas
Two twin series of carbon replicas were cast from SBA-15 by the acidcatalyzed precipitation polycondensation of various amounts of FA in
suspensions of the matrix, according to the modified procedure reported
in our former works [12,15,49]. The series differed in the liquid media
used for the incorporation of PFA into SBA-15 pores. These media were
selected in such a way to be significantly different in polarity and to be
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miscible with the monomer. For this purpose, water (dipole moment μ =
1.85 D) and toluene (μ = 0.36 D) (W- and T-series, respectively) were
chosen. The use of tartaric acid as a catalyst with lower acid strength
than in our earlier reports (hydrochloric acid) (for HCl pKa = − 6.3, while
for TA pKa1 = 2.98, and pKa2 = 4.34; each value given for water solution)
enabled the slower deposition of the polymer in the pores of SBA-15.
This prevented clogging the pores by the rapid growth of the polymer
plugs near the pore entrances.
In both series, the same intended monomer/silica mass ratios of 0.50,
1.10, 1.40, 1.70, 2.00, and 2.60 were adjusted using proper masses of
FA. TA was used as the polyreaction catalyst at the constant molar ratio
of TA/FA = 0.50. The cumulative mass of the solvent together with the
monomer was kept constant at 50.00 g for each synthesis batch. In the
case of the T-series, additionally, anhydrous sodium sulphate was added

as a desiccant agent at the constant molar ratio of Na2SO4/FA = 0.15 to
provide an anhydrous reaction environment. It traps the traces of water
originating from toluene and monomer impurities as well as this one
released in the FA polycondensation reaction. Briefly, an amount of
1.50 g of SBA-15 held before at 200 ◦ C overnight was added under
vigorous stirring (800 rpm) to a mixture of FA, solvent (water or
toluene), TA, and Na2SO4 (solely in the case of the T-series). The mixture
was placed in a two-neck round-bottom flask (100 cm3) immersed in an
oil bath placed on a magnetic stirrer and equipped with a reflux
condenser. It was then agitated at room temperature for 30 min, and
next a heating was turned on. After the temperature of the reaction
system reached 100 ◦ C, the mixture was isothermally held for the next
24 h under vigorous stirring (800 rpm). The resulting brownish com­
posite of poly(furfuryl alcohol) (PFA) and SBA-15 (PFA/SBA-15) was
then isolated, washed with distilled water or toluene (depending on the
reaction medium, respectively), and dried at 90 ◦ C overnight. After­
wards, to remove the TA and Na2SO4 (undissolved in the original
organic medium), the T-series materials were additionally washed with
an abundant amount of hot distilled water (~60 ◦ C) and dried again at
90 ◦ C. This step prevented the damage of the carbonizate structure
caused by its high-temperature oxidation with sodium sulphate during

branches of the nitrogen isotherms at p/p0 = 0.97–0.98. The micro- and
mesopore volumes (Vμ and Vme, respectively) were extracted from yintercepts of tangents fitted to αs plots within αs = 0.35–1.30 and
1.70–2.50 (SBA-15 matrix), αs = 0.50–1.00 and 1.50–2.40 (PFA/SBA-15
carbonizates), and αs = 0.60–0.85 and 1.70–2.80 (carbon replicas),
respectively. For the SBA-15 matrix and carbonizates, the foregoing
parameters were assessed with respect to the macroporous silica
LiChrospher Si-1000 (SBET = 25 m2 g− 1) [51], while for the ultimate
replicas, the non-porous carbon LMA10 was used as the reference [52].

The main pore diameters (Dp) were extracted from pore size distribution
curves (PSDs). In the case of SBA-15, the PSD was calculated using the
non-local density functional theory model (NLDFT; adsorption branch;
cylindrical pores assumption; software ASIQwin™ ver. 1.11, Quan­
tachrome Instruments), while for carbonizates and carbon replicas the
two-dimensional non-local density functional theory model devised for
carbons possessing heterogeneous surfaces was applied (2D-NLDFT;
SAIEUS software, ver. 3.0) [53,54].
Structural parameters were investigated by low-angle X-ray powder
diffraction (XRD) using a Bruker D2 Phaser instrument equipped with a
LYNXEYE detector. The XRD patterns were recorded using Cu Kα radi­
ation (λ = 1.54184 Å) in the angular range of 2θ = 0.80–4.00◦ with a
step of 0.02◦ .
Transmission electron microscopy (TEM) imaging was performed on
an FEI Tecnai TF20 X-TWIN (FEG) microscope operated at an acceler­
ating voltage of 200 kV. Before measurements, samples were dispersed
in isopropanol followed by sonication for 10 min and deposited onto
carbon-coated copper TEM grids by the drop-casting technique.
Mid-infrared spectra (300 scans each) were collected in the spectral
range of 650–4000 cm− 1 at a resolution of 4 cm− 1 using a Nicolet iS5
(Thermo Scientific) FT-IR spectrometer equipped with a DLaTGS de­
tector. A diffuse reflectance (DRIFT) device (EasiDiff™-Pike Technolo­
gies) and attenuated total reflectance kit (iD7 ATR Accessory, Thermo
Scientific) for solid and liquid samples analyses were used, respectively.
Prior to the measurements, the solid materials, held before at 105 ◦ C for
72 h, were diluted with spectral grade dry KBr to 2 wt% and gently
milled in an agate mortar, while the ATR spectra for the liquid samples
were acquired without dilution.
Average values of the ζ-potential (ZP) of SBA-15 immersed in pure
reaction media (water and toluene) and respective FA solutions, were

determined by using a Zetasizer Nano ZS instrument equipped with a
maximum 4 mW He–Ne laser, emitting at 633 nm (Malvern Instruments
Ltd., Malvern, U.K.). The measurements were carried out using a Uni­
versal dip cell (ZEN1002) combined with a glass cuvette (PCS1115).
Prior to the measurements, four suspensions containing 0.1 wt% of
freshly calcined SBA-15 were prepared using distilled water, toluene,
and corresponding 7.8 wt% solutions of FA. The suspensions were son­
icated in an ultrasonic bath for 15 min. The analyses were performed at
25 ◦ C. Before commencing the measurement, the sample’s temperature
was allowed to equilibrate in the instrument chamber for 2 min. Each
analysis was repeated three times.
X-ray photoelectron spectroscopy (XPS) measurements were per­
formed on a Prevac photoelectron spectrometer equipped with a hemi­
spherical analyzer (VG SCIENTA R3000) using Al Kα rays (E = 1486.6
eV) as an X-ray radiation source at a constant pass energy of 100 eV for
survey and high-resolution modes. The powder composites were placed
on a sample holder and introduced by a load lock into an analytical
chamber with base pressure of 5 × 10− 9 mbar. The binding energy scale
was calibrated using the Si 2p line of pristine SBA-15 silica at 103.6 eV.
The surface composition was analysed on the base of the areas and
binding energies of Si 2p, C 1s, and O 1s core levels. The spectra were
fitted using CasaXPS software version 2.3.23.
An adsorptive interaction of the silica surface with FA in an aqueous
medium was investigated employing total organic carbon (TOC) anal­
ysis using a Shimadzu TOC-VCPH apparatus. Briefly, 1.0000 g of freshly
calcined SBA-15 was immersed at room temperature (21 ◦ C) in 50.00 g
of a 1.00 wt% FA-water mixture in a 100 cm3 round-bottom flask

T

carbonization as follows: Na2 SO4 + 4C ​ → ​ Na2 S + 4CO↑ . The assynthesized composites were labelled as PFA/S-x_y, where x stands for
the real PFA/SBA-15 mass ratio (determined based on TG measurements
under an air atmosphere), and y refers to the series (y ≡ W and T for
water and toluene medium, respectively). Additionally, two samples of
bulky PFA were synthesized without using the silica matrix in water and
toluene following the same protocol as for the composites. These ma­
terials were labelled as PFA_W and PFA_T, respectively. The PFA/S-x_y
composites were carbonized in a tubular quartz furnace under an argon
atmosphere (40 cm3 min− 1) at 850 ◦ C for 4 h using a heating rate of β =
1 ◦ C min− 1. Finally, the silica matrix was removed by double etching
with HF at room temperature for 90 min. Namely, 1.00 g of carbonizate
was immersed in 30.0 cm3 of 5% HF solution and gently shaken ever and
again. The carbonizates and corresponding carbon replicas were marked
as C/S-x_y and C-x_y, respectively.
2.2. Characterization methods
Textural parameters of materials were investigated by means of lowtemperature adsorption-desorption of nitrogen (− 195.8 ◦ C). The iso­
therms were collected using an ASAP 2020 sorptometer (Micromeritics).
Prior to the analyses, the materials were evacuated at 250 ◦ C for 6 h
under vacuum. The specific surface areas (SBET) were calculated ac­
cording to the Brunauer–Emmett–Teller model within p/p0 = 0.05–0.20,
while the micropore surfaces (Sμ) were assessed based on the t-plot
model (using the de Boer equation) at the same relative pressure range.
The external surface areas of SBA-15 and carbonizates (Sex) were
computed from slopes of tangents fitted to αs plots within αs = 1.70–2.50
and 1.50–2.40, respectively. The total pore volumes (Vt) were computed
according to the single-point approach (s-p) from the adsorption
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polarity of the reaction medium and the FA/SBA-15 ratio used. Namely,
in the case of the toluene series, the yield of polymerization is roughly
two-threefold higher compared to that of the W-series (excepting the
materials with the highest polymer content). This is reflected in a similar
trend observed for the degree of filling of the pores, which for the Wseries varies between ca. 10 and 73%, while for the T-series it spans in
the range 28–68%. It is pertinent to mention that the lower FA content,
the higher polymerization efficiency is observed, notwithstanding the
reaction medium (excepting the PFA/S-1.30_W composite; we reported
on a similar effect in our previous works [50,70]). For the composites
with the lowest PFA loading, it attained 34 and 99% for the W- and
T-series, respectively. This means that the use of toluene as the reaction
medium facilitates definitely the successful incorporation of PFA to the
channels of the silica matrix. It is reasonable to conjecture that the
effectiveness of silica decoration with PFA is governed by the following
circumstances: (i) behavior of silica itself under harsh hydrothermal
conditions of PFA deposition (SBA-15 undergoes partial leaching fol­
lowed by re-precipitation of silica resulting in the flattening of the inner
surface corrugations [55]), (ii) state of the SBA-15 surface silanols in an
aqueous and anhydrous environment and their likely role in the
monomer pre-adsorption, (iii) mutual interaction between solvent and
monomer molecules, (iv) catalyst acidic strength in these media, and (v)
viscosity of the FA-solvent mixtures, which may play a crucial role in the
kinetic of infiltration of the matrix pore system with carbon precursor
[56]. Surprisingly, this parameter was not discussed in the literature as
far. Herein, we have measured the viscosity of the studied synthesis
systems. The kinematic viscosity of the 7.80 wt% FA-water mixture at
21 ◦ C is equal to 1.14 mm2 s− 1, whereas for the FA-toluene mixture of

the same concentration is 0.72 mm2 s− 1 (for pure FA it equals 4.73 mm2
s− 1). Thus, this may be a hint unraveling the higher PFA loading within
the T-series.
Interesting insights are provided by the analysis of the TG, DTG, and
DTA curves (see Fig. S1). Regardless of the reaction medium as well as
the polymer content, the materials feature similar decomposition pro­
files with two distinctive stages (Fig. S1, DTG profile) with the maxima
centered at 340 and 520 ◦ C. However, it is worth noting that in the DTG
curves recorded for the composites of the W-series, the high-temperature
maximum dominates, while the opposite situation is observed for the Tseries. This suggests a slightly higher thermal stability of the W-series
materials.
Furthermore, the differences in the manner of PFA deposition find
reflection in the macroscopic images of the materials. Namely, one can
clearly see also the differences in the colors of the as-made composites
(Fig. S2). The brighter tints of the W-series materials may be seen at a
glance even when the polymer content is higher than that one of the Tseries. On one hand, this is indicative of a higher level of T-series PFA
crosslinking. On the other hand, this suggests another mechanism of
polymer chain growth favoring the formation of chromophoric species
(conjugated π-bond systems) in the T-series [57–60]. It should be noted
that this in turn may influence the carbonization of the polymer and the
structural ordering of the final carbon material.

Fig. 1. Efficiency of FA polymerization and effectiveness of PFA deposition in
the pore system of SBA-15 expressed as true PFA/SBA-15 mass ratio and pore
filling degree (the shaded areas refer to the real PFA contents required for
obtaining the respective replicas).

equipped with a magnetic stirrer. Then, the suspension was vigorously
stirred (1000 rpm) for 30 min. After separation of silica by filtration on a
Büchner funnel, the filtrate was subjected to the TOC analysis. The

capability of silica towards monomer adsorption was estimated based on
a drop in the FA concentration during silica immersion compared to the
mother liquor.
High-resolution thermogravimetric measurements (TG) were carried
out using a SDT Q600 analyzer (TA Instruments). An amount of ca. 20
mg of a sample was heated in a corundum cup from 30 to 980 ◦ C (β =
20 ◦ C min− 1) at an air atmosphere (100 cm3 min− 1). The true amounts of
the carbon precursor incorporated into the silica matrices (i.e. the real
polymer/silica mass ratios in the PFA/SBA-15 composites) were calcu­
lated based on the mass loss related to the burning-off of the polymeric
component regarding to the mass of the silica residue. The silica’s pore
filling degree was computed as a ratio of PFA volume (density of bulky
PFA at room temperature, ρPFA = 1.55 g cm− 3 [50]) with respect to the
Vt of the silica matrix (expressed as a percentage). The same TG mea­
surement procedure was employed for the study on the
thermo-oxidative stability of the ultimate carbon replicas.
Kinematic viscosity of the binary mixtures of FA with water and
toluene was determined using a suspended-level (Ubbelohde) viscom­
eter. The measurements were carried out at 21 ◦ C for the mixtures
containing 7.80 wt% of the monomer. This concentration corresponds to
the mixtures used in the syntheses of the highest loaded composites.

3.2. Textural and structural characteristics of C/S-x_y carbonizates and
C-x_y replicas

3. Results and discussion

Textural and structural parameters of the parent silica matrix SBA-15
as well as the carbonizates and corresponding carbon replicas were
investigated by low-temperature adsorption of nitrogen and low-angle

X-ray diffraction. The collected isotherms together with the corre­
sponding PSDs are depicted in Fig. 2, while Fig. 3 displays the relevant
XRD patterns. The respective textural and structural parameters are
gathered in Table 1. For better readability, all results are presented
along with the ascending real polymer loading.
The N2 adsorption isotherm for pristine SBA-15 is a textbook
example of a IV(a) type with H1 hysteresis loop featuring steep parallel
adsorption and desorption branches (cf. Fig. 2) [61]. This evidences the

3.1. Effectiveness of PFA incorporation into SBA-15 mesochannels
The efficiency of deposition of PFA inside the SBA-15 mesopore
system was investigated by thermogravimetric measurements per­
formed under the oxidative atmosphere (i.e. air). The calculated yield of
polymerization as well as the true PFA/SBA-15 mass ratios and pore
filling degrees are presented in Fig. 1, while the recorded TG mass
changes together with DTG and DTA curves are displayed in Supple­
mentary information section (Fig. S1).
The FA polymerization effectiveness is evidently influenced by the
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Fig. 2. Nitrogen adsorption-desorption isotherms (A, B) and respective PSDs (A′ , B′ ) for C/S-x_y carbonizates (red lines and symbols) and corresponding C-x_y
replicas (black lines and symbols) of W-series (A, A′ ): x = 0.17 (a), 0.28 (b), 0.35 (c), 0.39 (d), 0.43 (e), 1.30 (f), and T-series (B, B′ ): x = 0.49 (a), 0.73 (b), 0.94 (c),
1.02 (d), 1.10 (e), 1.22 (f). For clarity, the PSDs were offset of 0.25 (A′ ), and 0.60 cm3 g− 1 nm− 1 (B′ ) each. (For interpretation of the references to color in this figure
legend, the reader is referred to the Web version of this article.)

presence of open-ended main mesopores uniform in diameter and
long-range ordering of the architecture thereof. These mesochannels are
accompanied by a minor fraction of micropores which act as inter­
connecting channels. The textural and structural parameters of the silica

matrix are coherent with the typical values reported for SBA-15 in
previously published papers [4,12,13,15,35,43,44,50,55]. A very
similar isotherm was recorded for the SBA-15@850 material. Indeed,
annealing at 850 ◦ C entailed the shrinkage of the structure, manifested
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Microporous and Mesoporous Materials 329 (2022) 111542

Fig. 3. Low-angle XRD patterns for C/S-x_y carbonizates (red lines) and corresponding C-x_y replicas (black lines) of W-series (A): x = 0.17 (a), 0.28 (b), 0.35 (c),
0.39 (d), 0.43 (e), 1.30 (f), and T-series (B): x = 0.49 (a), 0.73 (b), 0.94 (c), 1.02 (d), 1.10 (e), 1.22 (f). Reflections assignment: * ≡ (1 0 0), ^ ≡ (1 1 0), “ ≡ (2 0 0), # ≡
(2 1 0), $ ≡ (3 0 0). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

in narrowing mesopores by 0.8 nm and extinction of microporosity. This
in turn caused a drop in both SBET and Vt (cf. Table 1) [15,50].
In the case of the carbonizates of the W-series, except the material
loaded with the highest amount of PFA (viz. Fig. 2A–a-e), the deposition
of PFA followed by carbonization did not influence the nature of the
isotherm. The only differences are a slight shift of the hysteresis loop
towards lower relative pressures caused by the thermal shrinkage of the
SBA-15 structure during carbonization of the PFA/SBA-15, and a
gradual decrease in both specific surface area and total pore volume
with increasing PFA content (cf. Fig. 2A, Table 1). This is not surprising

in view of the progressive filling of the silica’s pore system with carbon.
Interestingly, notwithstanding the pore filling degree, the pore size of
the carbonizates remains roughly constant (ca. 5.7–5.9 nm; Fig. 2A’,
Table 1). The N2 adsorption isotherm for the carbonizate containing the
highest amount of the carbon precursor (Fig. 2A–f) changes into the H2
(a) type with a characteristic desorption branch closure point at p/p0 ≈
0.4. This points to the effect of cavitation of the adsorptive in partially
blocked mesopores [61]. This is clearly reflected in PSD, which reveals
the shift in the main mesopore size to ca. 5.2 nm (Fig. 2A’–f, Table 1).
Expectedly, the accumulation of carbonaceous material entailed a
gradual decrease in the SBET and Vt, while increasing in the micropore
volume. This is a cumulative effect of the development of inherent
microporosity in the carbonized PFA as well as the formation of

slit-shaped micropores between the carbon material and silica wall due
to their uncapping caused by discrepancies in the shrinkage effect dur­
ing carbonization [13–15,50].
The two W-series samples with the lowest PFA contents (viz. C0.17_W and C-0.28_W; Fig. 2A–a,b) exhibit the N2 adsorption isotherms
of type I(b), which is common for micro-mesoporous materials [61]. The
C-0.28_W material reveals additionally a narrow H4 hysteresis loop
typical of suchlike mixed-porosity solids. Indeed, both materials feature
relatively low total pore volumes of 0.23 and 0.29 cm3 g− 1 with 56 and
41% contributions of micropores, respectively (cf. Table 1). The
featureless XRD patterns for these carbons disclose the entirely disor­
dered structures thereof (Fig. 3A–a,b).
Other N2 adsorption isotherms of the W-series carbons may be
classified as IV(a) type with H2(b) hysteresis loops. For these samples,
the adsorption branches show the presence of two inflections in the
mesopore region (this is best seen in the case of the C-1.30_W replica),
which confirm gradual development of two individual mesopore sys­

tems appearing along with increasing PFA content (cf. Fig. 2A’–c-e).
Interestingly, simultaneous extinction of the microporosity is observed.
Considering the PSDs for C-1.30_W (Fig. 2A’–f), it is evident that the
primary mesopores originating from the removal of silica matrix walls
centered at 3.1 nm are accompanied by far broader ones at ca. 4.5–15.0
nm resulting from the coalescence of the adjacent pores of SBA-15,
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Microporous and Mesoporous Materials 329 (2022) 111542

Table 1
Textural and structural parameters of parent SBA-15, SBA-15@850, C/S-x_y carbonizates, and corresponding carbon replicas.
Sample
SBA-15
SBA-15@850
C/S-0.17_W
C/S-0.28_W
C/S-0.35_W
C/S-0.39_W
C/S-0.43_W
C/S-1.30_W
C-0.17_W
C-0.28_W
C-0.35_W
C-0.39_W
C-0.43_W
C-1.30_W

C/S-0.49_T
C/S-0.73_T
C/S-0.94_T
C/S-1.02_T
C/S-1.10_T
C/S-1.22_T
C-0.49_T
C-0.73_T
C-0.94_T
C-1.02_T
C-1.10_T
C-1.22_T
a
b

SBET (Sex)a [m2 g− 1]
886 (83)
642 (58)
517 (56)
524 (60)
464 (55)
465 (48)
455 (50)
302 (20)
385
441
732
755
927
855

282 (27)
344 (29)
284 (27)
372 (24)
302 (26)
69 (10)
1072
1779
2208
2033
1982
1222

Sμa [m2 g− 1]
34
0
0
4
18
24
28
101
261
223
109
178
167
9
13
53

34
103
49
4
280
0
0
0
0
73

Vtc [cm3 g− 1]

Vμa [cm3 g− 1]

1.15
0.85
0.76
0.79
0.71
0.66
0.64
0.27
0.23
0.29
0.83
0.71
0.99
1.04
0.35

0.40
0.30
0.35
0.30
0.05
0.63
1.20
2.12
1.94
1.87
1.17

0.03
0.00
0.00
0.00
0.01
0.01
0.01
0.05
0.13
0.12
0.07
0.11
0.11
0.02
0.00
0.02
0.01
0.05

0.02
0.00
0.20
0.07
0.00
0.00
0.00
0.06

Vme [cm3 g− 1]
a

1.02
0.78a
0.69a
0.71a
0.63a
0.60a
0.57a
0.20a
0.10b
0.17b
0.76b
0.60b
0.88b
1.02b
0.31a
0.34a
0.25a
0.27a

0.25a
0.04a
0.43b
1.13b
2.12b
1.94b
1.87b
1.11b

Dp [nm]
d

2.6; 7.6
3.0; 6.8d
3.9; 5.8e
3.9; 5.9e
3.9; 5.9e
3.9; 5.7e
3.9; 5.7e
5.2e
–
–
3.8; 5.5e
3.7; 5.5e
3.8; 5.5e
3.1; 6.9e
4.8e
4.7e
4.0e
4.1e

4.1e
–
–
2.9e
2.9; 4.1e
2.9; 4.1e
2.9; 4.0e
3.7e

Dw [nm]
f

3.1
2.7f
–
–
–
–
–
–
–
–
–
–
–
5.9g
–
–
–
–

–
–
–
–
1.4h
1.4h
1.4h
5.7g

a0i [nm]
10.7
9.5
10.0
9.8
9.9
9.7
9.7
9.8
–
–
–
–
–
9.8
9.5
9.6
9.8
9.8
9.7
–

–
–
9.8
9.8
9.7
9.5

αs model.

Vme = Vt ​ (s− p) − Vμ (αs) .
Single-point at p/p0 = 0.98.
d
NLDFT for silicas; adsorption branch; cylindrical pores assumed.
e
2D-NLDFT for carbons with heterogeneous surfaces.
f
Silica wall thickness; Dw,sil. = a0 − Dp .
(
)1 2
ρcarb. − 1 + Vμ
g
Carbon nanorod diameter; Dw,carb. = c⋅d1 0 0
, c – constant; for cylindrical pores c = 1.213; d1 0 0 – interplanar spacing; d1 0 0 = 2⋅ d2 0 0 ;
−
1
Vme + ρcarb. + Vμ
− 3
ρcarb. – amorphous carbon density; ρcarb. = 2.05 g cm [15,43,46].
c

/

h

Average thickness of carbon wall in the tube-type replicas; wC =

(Dsil.850
− Din
p
p )
, Dsil.850
is the mesopore diameter of SBA-15@850; Din
p
p means the inner diameter of
2

carbon tube.
i
Due to the featureless XRD patterns in the (1 0 0) reflection region (Fig. 3), the lattice parameters were calculated from (2 0 0) reflection; a0 = 4⋅ 3−

which were previously either entirely empty or partially filled with the
carbon precursor. The width of this peak should not be surprising given
the random distribution of PFA inside the SBA-15 pore system, which
yields the pseudo-CMK-3 structures [15,50]. Due to the defective struc­
ture, this material exhibits a slightly lower specific surface area than
typical CMK-3, while its relatively high total pore volume of 1.04 cm3
g− 1 is understandable. Noteworthy, despite the non-ideality of these
frameworks, their XRD patterns gradually take shape of the pattern of
standard CMK-3 material along with increasing PFA loading, achieving
the maximum similarity for the highest PFA content (Fig. 3B–c-f).

Recently, we reported on the formation of similar structures when
SBA-15 with a low degree of silica framework condensation was
employed as a hard template (therein, the silica matrix was detemplated
under mild conditions using an acidified solution of KMnO4), and the
deposition of PFA was carried out in water medium [15,50]. However, it
is pertinent to mention that using hydrochloric acid as a polyreaction
catalyst leads to the formation of the typical CMK-3 structure [12,49].
Another scenario was observed for the T-series carbonizates. The two
carbonizates with the lowest PFA loadings (i.e. C/S-0.49_T and C/S0.73_T) show the N2 adsorption isotherms of type IV(a) with H1 hys­
teresis loops (Fig. 2B–a,b) [61]. It is worth noting that these loops are
shifted to lower relative pressures compared to both SBA-15 and
SBA-15@850, which suggests a progressive cladding of the inner walls
of pores with the polymer. Indeed, considering the corresponding PSDs
(Fig. 2B’–a,b), a gradual decrease in the diameter of the main mesopores
with increasing PFA content is evident. The materials with moderate
PFA loadings (i.e. C/S-0.94_T, C/S-1.02_T and C/S-1.10_T) feature the

1/2

⋅ d2 0 0 .

isotherm of IV(a) type with a H2(a) hysteresis loop. For these samples,
the main pore size equals ca. 4.0–4.1 nm regardless of the real content of
the carbon precursor (Fig. 2B’–c-e). The carbonizate with the highest
PFA loading (C/S-1.22_T) shows the maximum nitrogen uptake close to
nil (Fig. 2B–f), which combined with the featureless PSD (Fig. 2B’–f)
clearly evidences its total pore filling with the polymer. The XRD pat­
terns collected for the T-series carbonizates show lower intensity of the
characteristic reflections compared to the parent silica, which proves
filling the pores of the hard template with organic material (Fig. 3B).

The analysis of the behavior of the N2 adsorption isotherms recorded
for the final carbon replicas of the T-series provides particularly inter­
esting conclusions. The replicas derived from the two materials with the
lowest PFA content disclose a micro-mesoporous character thereof
(isotherm of type I(b) with a H4 loop), similar to the corresponding Wseries replicas (see Fig. 2A–a,b, vs. Fig. 2B–a,b) [61]. The lack of a
long-range ordering of the architecture of these materials is visible in the
low-angle XRD patterns (Fig. 3B–a,b). Undoubtedly, a change is seen
when considering the OMCs synthesized from the carbonizates of
moderate PFA loadings (real polymer/silica ratio of 0.94–1.10, viz.
Fig. 2B–c-e). Namely, the isotherms are of type IV(a) with H1 hysteresis
loop and well-distinguished two inflections in the adsorption branch at
ca. p/p0 = 0.30–0.50, and 0.55–0.70. Apparently, this is reflected in the
respective PSDs displayed in Fig. 2B’–c-e, which show the bimodal
mesoporosity of these materials featuring two maxima centered at 2.9
and 4.0–4.1 nm, typical of hollow-type CMK-5 carbon replica (Table 1)
[18]. The narrower pores originate from the leaching of silica matrix
walls, while the broader ones are inherited from the carbonizate, in
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Microporous and Mesoporous Materials 329 (2022) 111542

Fig. 4. TEM images and Fourier diffractograms of carbon replicas: C-0.43_W (A), C-1.30_W (A′ ), C-1.10_T (B), and C-1.22_T (B′ ).

which the inner silica walls were covered with a PFA film (intra-tubular
carbon pores). Interestingly, the share of the latter one in the mesopore
volume decreases with increasing content of carbon precursor (see
Fig. 2B’–c-e, decreasing the maxima at 4.0–4.1 nm), while the pore

diameter stays constant. Indeed, the bimodal porosity contributes to the
exquisite development of the specific surface area exceeding 2200 m2
g− 1 (cf. Table 1). It should be underscored that such textbook examples
of CMK-5 isotherms and PSDs were rarely reported in the literature. The
XRD patterns of these materials (Fig. 3B–c-e) with five characteristic
reflections including the dominating (1 1 0) one are indicative of the
p6mm arrangement of the hollow-type carbon material [3,62]. Thus, an
excellent quality of the synthesized materials is evident. It may be

surprising that the T-series sample with the highest PFA loading yielded
a high-quality rod-type CMK-3 replica, notwithstanding its pore filling
degree barely equals 68% (see Figs. 1, Fig. 2B–f, Fig. 2B’–f). However,
given the thermal shrinkage of the SBA-15 structure during composite
carbonization, this is understandable (such shrinkage causes a reduction
in Vt roughly by ¼, cf. Table 1) [15,50]. This carbon replica features
monomodal mesopores of 3.7 nm in diameter, a total pore volume of
1.17 cm3 g− 1, and SBET of 1222 m2 g− 1. Such textural parameters are in
accordance with previous reports on CMK-3 materials [3,4,6,9,10,12,
13,15,35,43,49,50]. The structural ordering is manifested in the XRD
pattern with three distinguished reflections, also typical of suchlike
structures (Fig. 3B–f) [34].
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Microporous and Mesoporous Materials 329 (2022) 111542

Fig. 5. DRIFT spectra collected for the SBA-15 after pre-adsorption of FA from water (A), and toluene (B) solutions (red lines) and after contact with pure solvents
(black) followed by desorption at room temperature overnight (a), and at 50 ◦ C (b), 100 ◦ C (c), 150 ◦ C (d) for 1 h. The ATR spectra of pristine SBA-15, pure FA and

respective solvents are added in the bottom. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

The phase purity of the chosen carbon replicas was studied by highresolution thermogravimetric measurements at an air atmosphere [46].
The relevant results are displayed in Fig. S3. The narrower DTG curves
for the T-series evidence the higher homogeneity (i.e. lack of impurities
being disordered carbonaceous material, which could be formed onto
the external surfaces of silica matrix) of these materials compared to the
W-series. Interestingly, the later ones exhibit the same temperature of a
maximum oxidation rate (ca. 625 ◦ C) notwithstanding the PFA loading
in the materials. In contrast, the CMK-5 sample shows the maximum
combustion rate at the temperature of ca. 15 ◦ C lower. This is justified by
the open-work structure of this material.

mesoporosity and an excellent hexagonal arrangement (p6mm space
group). This is in line with the N2 adsorption isotherms and XRD pattern
(Fig. 2B–e, Fig. 2B’–e, Fig. 3B–e). As expected, the higher loading of
SBA-15 with PFA achieved in toluene results in the formation of a reg­
ular rod-type CMK-3 replica with a perfect hexagonal mesoscopic ar­
chitecture (Figs. 4B’, 2B–f, 2B’–f and 3B–f). It should be emphasized that
the analysis of the dozen TEM images of both materials from the T-series
(not shown here) disclosed a lack of the effect of the formation of an
external amorphous shell of the excessing PFA enveloping the PFA/silica
composite particles. Such a phenomenon was observed in the case of the
highest-loaded materials synthesized in water as was reported in our
previous works [12,15]. Naturally, this influences positively the quality
of the carbon replicas synthesized in toluene in terms of both structural
ordering and textural parameters.

3.3. Morphology of carbon replicas
The structural ordering and morphology of the carbon replicas were

investigated by TEM imaging. The micrographs taken for the chosen
materials of both series together with relevant Fourier diffractograms
are displayed in Fig. 4.
The images recorded for the carbon material based on the partially
filled W-series composite (Fig. 4A) reveal a poor ordering of the final
structure (cf. Fig. 3A–e), although the Fourier diffraction pattern dis­
closes vestigial hexagonal architecture features. This is coherent with
the textural parameters (see Fig. 2A–e, Fig. 2A’–e). The higher degree of
filling of the PFA matrix results in obtaining the pseudo-CMK-3 replica
[15,50]. As mentioned above, in this case, the carbon precursor fills the
honeycomb pore system of SBA-15 randomly, i.e. some channels remain
empty, while others are partially or completely filled with PFA. This may
be seen in Fig. 4A’. The darker and brighter regions correspond to car­
bon nanorods and cavities formed from empty pores, respectively.
Noteworthy, despite these structural discontinuities, such material is
mesoscopically well ordered (cf. Fig. 2A–f, Fig. 2A’–f, Fig. 3A–f).
More interestingly, the carbon structure derived from the partially
filled composite of the T-series displays a fabulous TEM image taken
along the [1 0 0] direction (Fig. 4B). This is typical of a high-quality
hollow-type CMK-5 replica with well-distinguished bimodal

3.4. Mechanism of PFA deposition: a spectroscopic study
The substantial differences in the textural parameters of the W- and
T-series OMCs were a premise suggesting different mechanisms of
deposition of the carbon precursor depending on the polarity of the
reaction medium. This inspired us to deepen the study on the in­
teractions of monomer and polymer with the silica surface. We put ef­
forts to unravel these issues by the investigation of FA-silica interactions
(FT-IR) and analysis of the non-carbonized composites (FT-IR and XPS).
3.4.1. Monomer pre-adsorption

The adsorptive interactions of silica surface with monomer and both
reaction solvents were studied by means of DRIFT spectroscopy. For this
purpose, the freshly calcined silica (0.30 g) was immersed in 20.00 cm3
of 5 wt% solutions of FA in water and toluene, respectively, at room
temperature for 3 h. Additionally, to distinguish the silica-solvent in­
teractions, the SBA-15 matrix was contacted in the same manner with
the pure solvents. After the contact, the materials were separated
without washing, dried at room temperature overnight, and then evac­
uated under static conditions at 50, 100, and 150 ◦ C for 1 h. The
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Microporous and Mesoporous Materials 329 (2022) 111542

adsorption capacity reached 0.29 μmol of the monomer per square meter
of the silica surface. Such a negligible monomer adsorption suggests the
preferential adsorption of water. This is in line with the above FT-IR
study as well as the reports published elsewhere [65].
Another scenario was observed when silica was immersed in pure
toluene and the FA-toluene mixture (Fig. 5B–a-d). The spectrum of the
sample after the contact with pure solvent followed by evacuation at
room temperature showed the complete loss of toluene. Thus, the state
of the freshly calcined silica surface was restored even for such mild
desorption conditions. The complete evaporation of the solvent at the
temperature of ca. 90 ◦ C below its boiling point (i.e. 110 ◦ C) indicates a
low affinity of toluene towards the silica surface. Indeed, the phobic
character of silica towards aromatics adsorption is not surprising [66].
In contrast, the contact of SBA-15 with the FA-toluene mixture clearly

revealed that the free silanols were involved in the attracting of the
alcohol molecules. This means that monomer adsorption is favored
when toluene is used. With this in mind, the formation of hollow-type
carbon replicas as a result of polycondensation of the FA selectively
adsorbed onto the silica surface appears understandable. Moreover, the
comparison of the intensity of FA bands adsorbed in water and toluene
(see Fig. 5A–a,d vs. Fig. 5B–a,d) confirms that the use of the aprotic
medium promotes the adsorption of larger amounts of alcohol, which in
turn is in line with the higher efficiency of PFA deposition in toluene (cf.
Fig. 1). These findings were also proven by the measurements of the zeta
potential of the parent silica immersed in pure reaction media and
FA-solvent mixtures. In contact with pure solvents, the SBA-15 silica
revealed typical ZP values (− 31.3, and − 24.2 mV for water and toluene,
respectively) [67,68]. In contact with the FA solutions, the surface be­
comes depleted in a negative charge; in the case of FA-water, the ZP
equaled − 6.7 mV, while for FA-toluene the ZP reached +12.9 mV. This
is due to the protonation of the free silanols by the FA molecules as
follows: ≡ Si − OH+
2 : OR [69].
3.4.2. Surface chemistry of PFA/silica composites
The DRIFT spectra of the as-made W- and T-series PFA/silica com­
posites with the highest PFA loading and respective bulk polymers are
shown in Fig. 6. The curve-resolved C 1s regions of the corresponding
XPS spectra are displayed in Fig. S4, while the concentrations of
particular carbon- and oxygen-containing surface moieties are gathered
in Table S1.
The spectrum recorded for the PFA/S-1.30_W composite is essen­
tially a simple superposition of the silica and bulk PFA spectra, excepting
the 2800–3750 cm− 1 region and the band at 979 cm− 1 (Fig. 6a) [70].
The decrease in free silanol band intensity (3745 cm− 1) accompanied by

the increase in the intensities of 2800–3750 cm− 1 and 962 cm− 1 modes
may be assigned to the profound rehydration of the silica surface during
the PFA deposition, which is not surprising taking into account its hy­
drothermal conditions (i.e. 100 ◦ C, 24 h). The identical shape of the
absorption modes of this composite and bulky PFA (Fig. 6a,b, respec­
tively) confirms the lack of chemical anchoring of the monomer mole­
cules before the polyreaction (i.e. the absence of the Si–O–C bridges that
should be expected in this case). This ultimately proves the shielding role
of water molecules occupying the adsorption sites on the silica surface.
The spectrum of the PFA/S-1.22_T composite discloses an increase in
the intensity of 2800–3750 cm− 1 band accompanied by the extinction of
the 3745 cm− 1 mode, while the intensity (and position) of the absorp­
tion at 979 cm− 1 remains unaltered (Fig. 6c). This suggests the
engagement of isolated silanols in FA anchorage while lacking matrix
rehydration, which is not surprising given the anhydrous conditions
provided in the reaction system. More interestingly, the PFA/S-1.22_T
material features the disappearance of the 3120 cm− 1 (− CH in furan
ring) as well as 1560 and 1600 cm− 1 bands (furan ring vibrations). This
is due to the effect of the acid-catalyzed furan ring-opening leading to
the formation of γ-diketone moieties, which is evidenced by the presence
– O species)
of an intense band at 1715 cm− 1 (stretching vibrations of C–

Fig. 6. DRIFT spectra of non-carbonized PFA/SBA-15 composites and bulk PFA
samples: PFA/S-1.30_W (a), PFA_W (b), PFA/S-1.22_T (c), and PFA_T (d). The
green line represents the spectrum of pristine SBA-15. (For interpretation of the
references to color in this figure legend, the reader is referred to the Web
version of this article.)

collected DRIFT spectra are gathered in Fig. 5.

The contact of the SBA-15 template with both pure water and the FAwater mixture resulted in rehydration of the silica surface. This is
manifested by a pronounced drop in the intensity of free silanols ab­
sorption at 3745 cm− 1 accompanied by a significant increase in the in­
tensity of a broad band at 2800–3750 cm− 1 ascribed to the stretching
vibrations of hydrogen-bonded silanols, and shift of the stretching Si–O
mode from 979 to 962 cm− 1 (stretching Si–OH) (Fig. 5A–a-d) [50,63]. In
the case of the silica immersed in FA-water mixture, the spectra reveal
additionally the features of FA, namely, at 2929 and 2873 cm− 1
(asymmetric and symmetric stretching of methylene bonds in –CH2–OH,
– C stretching in the furan ring), 913 (out-of-­
respectively), 1505 (C–
plane –CH deformation vibrations), and 744 cm− 1 (out-of-plane –CH
bending in furan ring) [64,65]. Interestingly, the characteristic band
assigned to strongly physically adsorbed water at 1625 cm− 1 shows a
lower intensity for the FA-water mixture. This is the effect of competitive
water-alcohol sorption as was reported elsewhere [63]. Thus, the silica
surface simultaneously attracts both water and FA with the engagement
of free silanols. However, the accessibility of adsorption sites for alcohol
molecules is largely hindered by the shield of preferentially adsorbed
water. The desorption at elevated temperatures resulted in gradual
extinction of the FA bands, but no surface dehydration was observed.
Indeed, it is impossible to reverse the silica hydration process under
these conditions.
The interaction between silica and FA before the polycondensation
reaction was proven by TOC analysis of the FA-water mixture after 30
min of contact with freshly calcined SBA-15. It was found that the
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Microporous and Mesoporous Materials 329 (2022) 111542

Fig. 7. Pictorial illustration of the postulated mechanism of PFA deposition in polar and nonpolar synthesis media and the structures of the resultant replicas.

Scheme 1. Factors governing the formation of carbon replicas depending on the synthesis conditions: the synthesis pathways verified herein and in our previous
reports [12,15,50].
11


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Microporous and Mesoporous Materials 329 (2022) 111542

[70]. Noteworthy, this effect was more profound for the T-series. As a
consequence of ring-opening reactions, the intensity of the 790 cm− 1
band ascribed to 2,5-disubstituted furan rings in the PFA chain de­
creases. These findings are in compliance with the XPS analysis (see
Fig. S4, Table S1) [64,71,72].
Particularly interesting is the effect of the emerging of new bands at
1123 and 1195 cm− 1 in the spectrum of the PFA/S-1.22_T composite
(both absent for the W-series composite). This suggests the grafting of
the polymer onto the silica surface by a Si–O–C covalent bond [73–76].
This is coherent with the scenario anticipated from the textural pa­
rameters of the carbon replicas (cf. Textural and structural characteristics
of C/S-x_y carbonizates and C-x_y replicas). Namely, given the covalent
anchoring of PFA when precipitated in toluene, the higher homogeneity
of the polymer covering the silica walls is plausible. This, in turn, leads
to unraveling the excellent replication fidelity within the T-series ma­
terials and justifies the feasibility of manufacturing hollow replicas using

a non-polar synthesis medium. Interestingly, the XPS spectra did not
reveal the presence of the Si–O–C moieties (Fig. S4, Table S1). Most
likely this is due to the polymer layer shielding the PFA-SiO2 interface.
It is pertinent to mention that in the spectrum of the PFA/S-1.22_T
composite, the intensity of the silica bands within the range of
1000–1300 cm− 1 is much lower compared to the PFA/S-1.30_W mate­
rial, although the latter one contains a higher amount of polymer. This
again supports the anticipated differences in the manner of polymer
deposition (see Fig. S2).
The scheme illustrating the proposed mechanism of PFA precipita­
tion from polar and nonpolar media and the ultimate carbon structures
are shown in Fig. 7.

deposition of a polymer directly on the silica-solution interfacial.
Furthermore, the growth of polymer chains is not disturbed as it may
occur, for instance, in the case of impregnation methods (evaporation of
the solvent causes local fluctuations in the monomer concentration),
thus creating the opportunity to ideally clad the matrix surface. Sur­
prisingly, the level of silica matrix condensation and its pore diameter
turned out to be not so determinative in this regard. The results reported
herein gave rise to anticipate that the use of nonpolar reaction medium
and mildly acidic conditions may pave the way to develop a facile and
versatile method for the synthesis of other ordered carbon
mesostructures.
4. Conclusion
This study was aimed at unraveling the true role of the reaction
medium used for the nanoreplication of SBA-15 by the acid-catalyzed
precipitation polycondensation of FA in the SiO2 matrix suspension.
For this purpose, two twin series of carbon replicas were synthesized by
using water and toluene as dispersion media. The comprehensive

investigation of the textural and structural parameters, as well as the
morphology of the polymer/silica carbonizates and respective replicas,
disclosed that the polarity of the reaction medium plays a crucial role in
the deposition of the polymer onto the SBA-15 surface. Namely, in the
polar solvent the polymer chains start propagating radially from the
bulk monomer solution to the silica pore wall, while in the case of the
nonpolar medium their growth occurs in the reverse direction. This is
due to the competitive monomer-solvent adsorption onto superficial
SBA-15 silanols. In the case of the aqueous system, H2O molecules are
adsorbed preferentially forming a shield, which hinders the formation of
a homogenous PFA layer, thus precluding the formation of a hollow-type
replica. Contrarily, when using the toluene-FA mixture, the monomer
adsorption is favored. The FA molecules anchor to the silica surface
covalently and clad it evenly, therefore facilitating the formation of an
excellent quality CMK-5 structure, rarely reported in the literature. This
finding may be a cornerstone to the development of simple and universal
method for the synthesis of other OMCs. This issue will be the subject of
our forthcoming research.

3.5. Other parameters affecting the structure of carbon replicas
Combining the present research with our former findings [12,15,50]
deeper insight into the influence of the synthesis conditions on the
structure of the ultimate carbon replicas of SBA-15 can be drawn.
Namely, the following parameters have been investigated as far: (i) pore
width of the SBA-15 matrix, (ii) degree of condensation of silanol groups
of silica, (iii) acid strength of polycondensation catalyst, and (iv) po­
larity of the reaction medium. All these parameters were examined for
various degrees of pore filling in SBA-15 with PFA. The verified mech­
anisms of carbon replicas formation depending on the combination of
these parameters are depicted in Scheme 1.

The combination of using the silica matrix with broader pores and
low degree of silica condensation (SBA-15 detemplated under mild
conditions using an acidified solution of KMnO4 without further calci­
nation [50]) accompanied by employing the high polarity medium
(water) and strong acid as a polycondensation catalyst (hydrochloric
acid) led to the formation of the pseudo-CMK-3 structures notwith­
standing the real PFA loading (see Scheme 1, the green dash path). A
similar scenario was observed in the present research for the route
involving the use of highly condensed (calcined) silica featuring nar­
rower mesochannels, which was decorated with the carbon precursor in
water under weak acidity (TA) (Scheme 1, green solid line path).
Interestingly, modifying the latter route by using a strong acid (HCl)
catalyst allows achieving a regular CMK-3 replica at higher PFA loading,
although moderate amounts of polymer still yield pseudo-CMK-3 struc­
tures (Scheme 1, blue dash path) [12,15]. Finally, it was found (in the
present study) that employing toluene instead of water while replacing
HCl with TA results in the formation of excellent structures of CMK-5 for
moderate polymer loadings and CMK-3 replica for the complete pore
filling with PFA (Scheme 1, red solid line).
Given these findings, one may conjecture that the two substantial
parameters driving the mechanism of PFA deposition and, consequently,
tailoring the structure of carbon replicas, are the polarity of the PFA
precipitation medium and the acidity of the polycondensation catalyst.
Indeed, it is beneficial to perform the PFA incorporation under anhy­
drous conditions using a mild acid catalyst, which favors even

CRediT authorship contribution statement
Rafał Janus: Conceptualization, Synthesis, Characterization,
Writing – original manuscript, review & editing, Visualization, Formal
´­

analysis, Founding acquisition, Project administration. Piotr Natkan
ski: Characterization, Writing – original manuscript, review & editing.
Mariusz Wądrzyk: Writing – original manuscript, review & editing.
Marek Lewandowski: Writing – original manuscript, review & editing.
Piotr Łątka: Characterization. Piotr Ku´strowski: Writing – original
manuscript, review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
The research was carried out using the infrastructure of the AGH
Centre of Energy, AGH University of Science and Technology, as well as
´w. The latter
the Faculty of Chemistry, Jagiellonian University in Krako
one was partially purchased thanks to the financial support of the Eu­
ropean Regional Development Fund in the framework of the Polish
Innovation
Economy
Operational
Program
(contract
No.
POIG.02.01.00-12-023/08). Part of the experiments was carried out
thanks to the financial support of the National Science Centre in Poland
under the grant No. 2020/04/X/ST4/01697. R.J. acknowledges the
12


R. Janus et al.

Microporous and Mesoporous Materials 329 (2022) 111542

AGH University of Science and Technology for the financial support
within the subsidy No. 16.16.210.476.

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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2021.111542.
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