Sulfonated covalent triazine-based frameworks as catalysts for the hydrolysis of cellobiose to glucose

Article abstract of DOI:10.1039/c8ra04254c

Covalent triazine-based frameworks (CTFs) were synthesized in large scale from various monomers. The materials were post-synthetically modified with acid functionalities via gas-phase sulfonation. Acid capacities of up to 0.83 mmol g^-1 at sulfo

Full text of DOI:10.1039/c8ra04254c

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Sulfonated covalent triazine-based frameworks as
catalysts for the hydrolysis of cellobiose to
glucose†
Cite this: RSC Adv., 2018, 8, 22392
a
a
a
a
Jens Artz,
Irina Delidovich,
Moritz Pilaski, Johannes Niemeier,
a
b
a
Britta Maria Kubber, Khosrow Rahimi and Regina Palkovits
*
¨
Covalent triazine-based frameworks (CTFs) were synthesized in large scale from various monomers. The
materials were post-synthetically modified with acid functionalities via gas-phase sulfonation. Acid
ꢀ1
capacities of up to 0.83 mmol g at sulfonation degrees of up to 10.7 mol% were achieved. Sulfonated
CTFs exhibit high specific surface area and porosity as well as excellent thermal stability under aerobic
ꢁ
conditions (>300 C). Successful functionalization was verified investigating catalytic activity in the acid-
ꢁ
catalyzed hydrolysis of cellobiose to glucose at 150 C in H
2
O. Catalytic activity is mostly affected by
porosity, indicating that mesoporosity is beneficial for hydrolysis of cellobiose. Like other sulfonated
materials, S-CTFs show low stability under hydrothermal reaction conditions. Recycling of the catalyst is
challenging and significant amounts of sulfur leached out of the materials. Nevertheless, gas-phase
sulfonation opens a path to tailored solid acids for application in various reactions. S-CTFs form the basis
for multi-functional catalysts, containing basic coordination sites for metal catalysts, tunable structural
parameters and surface acidity within one sole system.
Received 18th May 2018
Accepted 12th June 2018
DOI: 10.1039/c8ra04254c
rsc.li/rsc-advances
6
diffusion limitation and thus, comparably low catalytic activity.
Introduction
To overcome such pore diffusion limitations, the sulfonation of
Acids play an important key role in chemical synthesis. macroporous polymers has been studied intensively. In this
Numerous essential organic reactions such as esterication or regard, especially polystyrene-based materials gained a lot of
8
addition reactions are carried out in the presence of acids. Due to attention. Unfortunately, thermal stability in comparison to
its versatile applications, sulfuric acid is the chemical with the zeolites is relatively low and desulfonation takes place at 260–
1
ꢁ
ꢁ
9,10
highest production output (170 Mio. t/a) worldwide. Therefore, 300 C in the gas or at 120–150 C in the liquid phase. Due to
this characteristic number was even utilized to describe the their ecological balance, solid acids are oen used in “green”
2
degree of industrialization of individual countries. Regardless of chemical processes, for example in the production of bio-
11,12
the inexpensiveness of these acids, their separation is a serious diesel.
Solid acids can be used as catalysts for both the
challenge in industrial processes and only possible via neutrali- transesterication of triglyceride in the classical bio-diesel
zation. The resulting non-recyclable salt load leads to high production as well as in the hydrolysis of biomass to desired
12–15
amounts of waste, thus causing a strong environmental impact. bio-diesel-like compounds.
Substitution of homogeneous acids by solid acids facilitates
In this respect, hydrolysis of water-insoluble cellulose to
the separation process strongly. Thus, solid acids can already soluble glucose attracts great attention as the most challenging
3
1
,4,5
be found in several industrial applications.
The most step for the production of bio-ethanol, bio-HMF and other
16
commonly used solid acids are zeolites, crystalline microporous value-added products. Herein, we focus on a soluble dimer of
alumosilicates with extremely high thermal stability (700–1300 glucose, cellobiose, representing a structural unit of cellulose.
ꢁ
4,6,7
C), tunable acidity and acid strength.
zeolites microporosity can be an obstacle, resulting in pore catalysts for hydrolysis of cellobiose into glucose. Inorganic
Nevertheless, the Vilcocq et al. has recently reviewed the activity of different solid
17
17
18
materials, such as zeolites or metal oxides, exhibited
moderate catalytic activity and selectivity for glucose formation.
a
Chair of Heterogeneous Catalysis & Chemical Technology Institut f u¨ r Technische und The best catalytic performance was demontrated by sulfonated
Makromolekulare Chemie, RWTH Aachen University Worringerweg 2, 52074 Aachen,
19
20
21
materials, including resins, carbons, silicas or organo-
Germany. E-mail: palkovits@itmc.rwth-aachen.de
b
22
silicas: up to quantitative yield of glucose can be attainable
DWI Leibniz-Institut f u¨ r Interaktive Materialien Forckenbeckstr. 50, 52074 Aachen,
over these catalysts. However, stability of the materials against
leaching of sulfur under reaction conditions appears to be the
Germany
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra04254c
21–23
main challenge for reusability of the catalysts.
At the same
1
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time, other structural changes of the materials were reported.
For example, Zhao et al. observed elimination of oxygen-
containing groups from the surface of the sulfonated graphite
24
oxide used as catalyst for cellobiose hydrolysis. Therefore,
water-tolerant acidic materials are of great interest for the
hydrolysis reaction.
In this regard, covalent triazine-based frameworks (CTFs;
Scheme 1) full all necessary requirements for a solid acidic
2
5,26
catalyst resulting aer sulfonation.
4
Thermally stable up to
ꢁ
00 C and chemically robust under relatively harsh reaction
conditions, these materials have drawn the interest of many
27–29
researchers as solid catalyst supports.
Additionally, they
possess sufficient benzylic structure elements for a feasible
sulfonation, while access to a large variety of commercial dini-
trile monomers grants control of the resulting materials Scheme 2 Monomers applied in the CTF preparation.
29
porosities and specic surface areas.
The sulfonation of aromatic systems is an electrophilic
aromatic substitution which in general occurs in the liquid
coulombic efficiencies and cycle lifetimes. However, the porous
character of the materials was entirely lost.
30–32
phase in presence of oleum or highly concentrated H SO .
2
4
Herein, we present a novel approach to obtain solid acid
catalysts with controllable structural properties based on CTFs.
CTFs obtained from a variety of dinitrile monomers (Scheme 2)
have been synthesized in large scale, resulting in materials with
the desired structural properties, namely porosity, specic
surface area and nitrogen content. The inuence of these struc-
tural properties on the gas-phase sulfonation method has been
studied intensively. Subsequently, the catalytic activity in the
hydrolysis reaction of cellobiose to glucose was investigated with
regard to the catalysts acidity, porosity and specic surface area.
To perform sulfonation reactions of polymeric materials under
milder conditions, the treatment of a solid sample with SO in
In this manner, sole surface
3
33,34
the gas-phase is possible.
adsorption of the acid groups can be suppressed, while covalent
bonding becomes favoured. First attempts to implement
sulfuric species within CTF materials were described by Bai
35
et al. Utilizing thiourea and thiosemicarbazide during a poly-
merization step with cyanuric chloride resulted in sulfur con-
taining CTF materials with high sorption capacities and
selectivities for uranium(VI). More recently, Talapaneni et al.
2
substituted ZnCl by elemental polymeric sulfur to enable
a solvent and catalyst-free polymerization of 1,4-dicyano-
Experimental section
Preparation of the CTF materials
36
benzene to the corresponding CTF. This way, a homogeneous
distribution of sulfur at high contents of 62 wt% was achieved.
The resulting material exhibited excellent properties as
a cathode material in Li–S batteries with superior initial
For the synthesis of the CTF-a material, 1,3-dicyanobenzene
(3.00 g, 23.4 mmol, 1 eq.) and ZnCl (15.96 g, 117.1 mmol, 5 eq.)
2
Scheme 1 (a) Idealized synthesis of a CTF based on 1,4-DCB as a monomer (CTF-c). (b) Proposed sulfonation methodology resulting in the
sulfonated CTF material (S-CTF-c).
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were mixed and ground together within a glovebox, transferred Characterization of the CTF and S-CTF materials
into a quartz ampule (12 cm height and 3 cm diameter) and
CHNS elemental analysis was performed on a PerkinElmer®
400 Series II CHNS/O Elemental Analyzer and supported by
measurements carried out at the microanalytical laboratory
Kolbe (M u¨ lheim an der Ruhr, Germany). ICP-OES analysis was
conducted with a SPEKTROFLAME ICP-D instrument from
SPEKTRO-Analytical Instruments. The samples (0.03 g) were
dried in vacuum for at least 3 hours. The ampule was then
ame-sealed and placed inside a furnace for 10 hours of heat
2

ꢁ
ꢁ
treatment at 400 C and further 10 hours at 600 C (heating rate:
1
ꢀ
1
0 K min ). Aer cooling to room temperature, the ampule was
broken open (CAUTION: the ampules are under pressure, which
is released during opening) and the solid product was ground
and washed thoroughly with water and diluted HCl (0.1 M). The
solid material was then ground in a ball mill (Fritsch Pulveri-
sette 23, 5 min, 30 Hz) to obtain a black powder, which was
molten in the presence of KNO (0.12 g) and KOH (1 g) within
3
a porcelain crucible and dissolved in aqueous HCl. For titration
of the sulfonated CTFs, samples (0.05 g) were dispersed in
1
1
0 mL of saturated NaCl solution (aq.) and stirred (500 rpm) for
week before titration to guarantee complete ion exchange. The

nally washed successively with water, diluted HCl, diluted
NaOH, water and THF. Aerwards, it was dried in vacuum for at
titrations were then carried out using a HI 9124 pH-meter from
HANNA INSTRUMENTS and 0.005 M NaOH (aq.). Thermogra-
ꢁ
least 12 h at 60 C. Materials based on 2,6-pyridinedicarboni-
0
trile (CTF-b), 1,4-dicyanobenzene (CTF-c) and 4,4 -biphenyldi-
ꢀ1
vimetric analysis (TGA) was carried out in air (60 mL min ) on
carbonitrile (CTF-d) were synthesized as described for 1,3-DCB
a Netzsch Simultaneous Thermal Analyzer Type STA449G
(3.00 g of monomer, 5 eq. of ZnCl
2
). As the lling degree of the
ꢀ1
Jupiter instrument at a heating rate of 10 K min . Nitrogen
ampule apparently has no effect on the physical parameters of
the resulting polymer, the ampule was charged to a maximum
of half ampule volume to prevent bursting within the furnace.
physisorption experiments were conducted on a Micromeritics
ASAP 2010 instrument. Samples were degassed for at least 15 h
ꢁ
at 150 C using a FloVacDegasser. Static volumetric measure-
ꢁ
ments were carried out at ꢀ195.8 C. The empty volume of the
cell was determined with helium. The specic surface area was
determined with the Brunauer–Emmet–Teller method (BET)
Gas-phase sulfonation of the materials
Gas-phase sulfonation reactions of all materials have been using data points at a relative pressure p/p
carried out in a glass reactor equipped with a glass frit 0.3. The total pore volume was determined at a relative pressure
between 0.05 and
0
34
according to literature (Fig. S8†). Aer evacuation of the of 0.98. The pore size distribution was calculated via Micro-
sample for at least 30 min (tap 1 closed), the CTF material (1.0 g) Active (version 1.01) using the density functional theory (DFT)
is exposed to SO
3
gas (tap 1 opened). The SO
3
gas is withdrawn
2
N -model for slit geometry at optimal Goodness of Fit vs. Reg-
from a reservoir (equipped with a stirring bar, 300 rpm) lled ularization (0.01) values for both RMS Error of Fit and Rough-
with 20% oleum (10 mL) via dynamic vacuum (p < 0.035 mbar). ness of Distribution. The cumulative pore volume at the pore
Within the reactor, the SO3 reacts with the CTF sample at width of 2 nm was used to determine the micropore volume of
ambient temperature. Aer 6 h tap 1 is closed and vacuum is the samples. Scanning electron microscopy (SEM) images were
applied to the sulfonated sample for further 30 min. The recorded on a Hitachi SU9000 Ultra-high resolution microscope
material is suspended carefully in water, ltered and washed equipped with a cooled Si(Li) X-ray detector Oxford Inca. The
thoroughly with water until a neutral pH value of the washing acceleration voltage was set to 20 kV. SEM samples were
solution is attained. The S-CTF sample is then dried within prepared by adsorbing dry powder on a Cu/lacey carbon grid
ꢁ
a furnace (at 80 C, ambient pressure) and a vacuum furnace (at and shaking off any loose material. Transmission electron
ꢁ
6
0 C) for at least 12 h.
microscopy (TEM) images were recorded on a Zeiss Libra 120
microscope. TEM samples were prepared by adsorbing dry
powder on a Cu/lacey carbon grid and shaking off any loose
material.
Hydrolysis of cellobiose to glucose with S-CTF
In a typical experiment cellobiose (0.5 g, 1.5 mmol), catalyst
(
0.05 g) and 4.5 mL water were charged into a stainless steel
Results and discussion
CTF preparation
autoclave with a gas inlet. The autoclave was sealed, pressurized
to 30 bar of nitrogen and heated to 110–150 C under stirring at
ꢁ
750 rpm. Aer the experiment, the autoclave was cooled down The synthesis of various CTF materials with different structural
in an ice bath and depressurized. The catalyst was ltered off properties was conducted using 1,3-dicyanobenzene (1,3-DCB),
using a syringe lter (CHROMAFIL® Xtra, PA-20/25, 0.20 mm) 2,6-dicyanopyridine (2,6-DCP), 1,4-dicyanobenzene (1,4-DCB)
0
0
and the reaction solution was analyzed via HPLC (Shimadzu and 4,4 -biphenyldicarbonitrile (4,4 -DCBP) as monomers
020) using two successfully connected organic acid resin (Scheme 2). As described elsewhere, bimodal microporous and
columns (100 mm ꢂ 8.0 mm and 300 ꢂ 8.0 mm) as previously mesoporous materials containing numerous N moieties are
2
45
2
reported. For recycling studies (aer 2 h reaction time) the accessible via ionothermal synthesis in molten ZnCl acting as
25
catalyst (0.075 mg) was ltered off via a Whatman™ ltration a solvent and a Lewis-acidic catalyst. Since ZnCl furthermore
2
system equipped with Anodisc 25 (0.20 mm) membranes, thor- acts as a porogen, an excess ZnCl /monomer molar ratio of 5 : 1
2
oughly washed with water and dried overnight in vacuum at was applied. Sequential heating of the monomer/salt mixture
ꢁ
ꢁ
6
0 C.
for 10 h each to 400 and 600 C in quartz ampules leads to the
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formation of an amorphous and partially carbonized framework is observed for CTF-b (Table 1, entry 4 and 5). This can be
containing tunable amounts of N with exceptionally high assigned to the dense coordination sphere during polymeriza-
porosity and specic surface areas.
tion, which is caused by coordination of the pyridine backbone to
In prior studies, quartz ampules of small volumes (l ¼ 12 cm, the Lewis-acidic ZnCl regardless of the synthesis scale. A similar
2
B ¼ 1.5 cm) have been used to guarantee sufficient heat transfer trend can be observed for CTF-c based on 1,4-DCB, which
29
during the polymerization reaction. Unfortunately, the small exhibits a nearly unchanged specic surface area and pore
ampule volume restricts to small quantities of the desired volume regardless of the production scale (Table 1, entry 8 and 9).
material and therefore the synthesis becomes both cost and time Interestingly, the specic surface area for CTF-d increases while
intensive. To tackle this challenge, ampules of larger volume (l ¼ the total pore volume decreases slightly in large scale synthesis,
1
2 cm, B ¼ 3.0 cm) were used in the present study and result in which can be assigned to favored partial carbonization as re-
approximately the 5-fold quantity of the desired product (4-fold ected by the lower amount of N within the resulting polymer
ampule volume increase combined with increasing lling (Table 1, entry 11 and 12). In both cases, the rigid structure of the
height). This enabled increasing the CTF yield from 0.6 g to para-substituted monomers seems to play an important role
approximately 3.0 g per batch. Properties of the CTFs synthesized during the formation of the porous framework. All four materials
in small and large ampules are summarized in Table 1. The have been synthesized repeatedly to investigate the reproduc-
2
resulting CTF materials were investigated via N physisorption as ibility of the large scale synthesis method (Table S1 and Fig. S7†).
well as elemental and thermogravimetric analysis (Table 1, Only minor changes in specic surface areas and total pore
Fig. S1–S5†). The materials obtained show the characteristic N2 volume could be detected and the course of the isotherms was
physisorption isotherms comparable to the previously described unchanged.
29
CTFs synthesized in small scale. In this regard, N2 phys-
The N content of the resulting CTFs is slightly lower for all
isorption isotherms of CTFs based on 1,3-DCB (CTF-a, Fig. S1†), materials synthesized in larger scale, except for CTF-b. This
0
1
,4-DCB (CTF-c, Fig. S3†) and 4,4 -DCBP (CTF-d, Fig. S4†) corre- seems to correspond to a slightly favored carbonization under
spond to type IV isotherms, emphasizing the mesoporous these reaction conditions. Nevertheless, reproducibility tests for
structure of all materials. While the hysteresis for both CTF-a and material synthesis (Table S1†) delivered comparable results for
-
c is barely existent, CTF-d exhibits a strongly pronounced CHN-analysis, conrming the N contents reported in prior
29
hysteresis according to the signicantly increased porosity, as it studies. In summary, N contents within the resulting mate-
would be expected from the structural properties of the mono- rials follow the same trend as reported for small scale ampule
mer. Regardless of the synthesis scale, a material based on 2,6- synthesis and can be correlated to the amount of nitrogen
DCP (CTF-b, Fig. S2†) presents a type I isotherm characteristic for available in the used monomers with slight deviations due to
completely microporous materials. The most drastic decrease in partial carbonization.
porosity and specic surface area when applying large scale
synthesis is found for CTF-a based on 1,3-DCB (Table 1, entry 1
and 2). This might be correlated to the incontrollable orientation Sulfonation in the gas-phase
of the meta-substituted nitrile-function during polymerization,
thus leading to unregulated structural parameters. In contrast,
material, resulting in an acidic catalyst. By introducing acid
only a slight decrease in specic surface area and microporosity
Via sulfonation, acid sites can be introduced into the solid
sites, both polarity and hence hydrophilicity will be increased.
Table 1 CHN elemental analysis, specific surface area, total pore volume of untreated and sulfonated CTF materials as well as the sulfur content
after modification with SO
3
in the gas-phase
a
a
a
b
2
ꢀ1
c
3
ꢀ1
a,d
No.
Material
C [wt%]
H [wt%]
N [wt%]
S
BET [m g
]
V
P
/VP(micro) [cm g
]
S
[wt%]
e
1
2
3
4
5
6
7
8
9
CTF-a
72.7
71.2
—
48.8
59.1
—
2.8
2.2
—
3.9
3.1
—
9.5
7.9
—
17.2
17.6
—
2439
1840
1509
1179
972
1.96/0.47
1.15/0.44
0.86/0.40
0.64/0.64
0.54/0.38
0.33/0.24
0.37/0.27
1.36/0.43
1.37/0.44
0.65/0.37
2.63/0.30
1.54/0.30
1.24/0.26
—
—
1.18
—
—
2.67
0.72
—
—
1.27
—
CTF-a
S-CTF-a
e
CTF-b
CTF-b
S-CTF-b
S-CTF-b*
603
699
—
—
—
e
CTF-c
CTF-c
73.4
62.1
—
84.9
75.5
—
2.2
2.5
—
1.7
4.3
—
10.4
6.6
—
3.7
1.3
—
2071
2001
1206
1683
1859
1561
10
11
12
13
S-CTF-c
e
CTF-d
CTF-d
S-CTF-d
—
1.57
a
b
c
Determined with elemental analysis. Specic surface area identied by Brunauer–Emmet–Teller (BET) method. Total pore volume determined
d
at p/p
0
¼ 0.98 and micropore volume determined via N
-DFT model. Determined via EA aer sulfonation with SO . A reference of non-sulfonated
2 3
e
29
CTF contained <0.01 wt% of sulfur. Material obtained via small scale synthesis as described in prior studies. With S-CTF-b* as reproduction of S-
CTF-b.
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Generally, the sulfonation of materials can be carried out in the other S-CTF materials showed comparable loadings as found for
liquid or the gas phase with sulfuric acid or oleum as a sulfo- the rst experiments aer extensive washing with an excess of
3
0–34
nation agent.
Prior studies could not conrm the sulfona- water for a prolonged time period (Table 1, entry 7).
27
tion of CTF materials in the liquid phase. Therefore, the
The acidity of all samples was determined via titration, ICP-
sulfonation of the CTFs investigated in the present study was OES analysis aer digestion of the S-CTF and elemental analysis
carried out in the gas-phase as it was presented by Richter (EA). All analysis techniques give comparable results for the
3
4
et al., using 20% oleum (20 wt% of SO
3
in sulfuric acid). acidity of the S-CTFs being approximately between 0.4 and
ꢀ1
Applying a dynamic vacuum, SO is evaporated from a reservoir, 0.8 mmol g (Table 2). The results of all the methods were in
3
passing through the material exhibited within a glass reactor. line with each other. The S-content determined via EA was used
The apparatus for the sulfonation is depicted in the ESI to calculate the sulfonation degree for all samples. The degree
(
Fig. S8†). The sulfonation in the gas-phase takes place under of sulfonation was calculated for the entire material, assuming
mild conditions at room temperature and enables easy work-up, complete and ideal polymerization of the corresponding
while sole adsorption of H SO from the liquid phase on the monomer.
materials surface is suppressed. In general, the sulfur loading does not follow a particular
The sulfonation of all CTF materials leads to a decrease in trend but all S-CTF materials contain around 1.0–1.5 wt% of
2
4
ꢀ1
both porosity as well as specic surface area (Table 1, entries 3, sulfur (ꢃ0.4 mmol g ) at a sulfonation degree in-between 5–
6, 7, 10 and 13), which cannot only be explained by the 10% on average. This could also be shown via reproducibility
increasing weight aer introduction of the sulfonic acid group. tests, which conrm only small discrepancies when sulfonating
More likely, the introduction of sterically demanding –SO H the same CTF type under identical conditions (Table S1†). The
3
groups aer sulfonation is responsible for the blockage of the degree of sulfonation for CTF materials is relatively low when
2
pore system. The course of N physisorption isotherms of the compared to commercially available solid acids. E.g. the degree
sulfonated materials (referred to as S-CTF) shows a comparable of sulfonation for an amberlyst resin with ion-exchange capac-
ꢀ
1
trend to the untreated materials, indicating that the porous ities of 4.8 meq g is 10-fold higher in comparison to the S-CTF
structure stays intact even aer treatment (Fig. S1–S4†). The materials presented in the present study. On the one hand, this
pore size distribution calculated for all S-CTF materials reveals might reect the high amount of triazine and pyridine moieties
a loss of porosity mainly in the mesoporous range for bimodal and their electronic properties hindering the sulfonation of
samples, while the micropores are mostly unaffected. These every single monomer. Especially the high degree of carbon-
structural ndings could be reproduced excellently for all four ization leads to a reduction of the aromatic functionalities
CTFs (Table S1 and Fig. S7†).
suitable for sulfonation. Hence, there is no idealized polymer
TEM images of a sulfonated S-CTF-d in comparison to structure available as it is for dened polymers such as poly-
untreated CTF-d do not reveal any signs of degradation or styrene in the case of Amberlyst® resins. On the other hand, the
decomposition (Fig. 1). Furthermore, SEM/EDX mapping partially carbonized regions of the CTF materials might have
images of S-CTF-d conrm sulfur evenly distributed throughout been sulfonated, meaning that the amount of xated SO
3
H-
the whole CTF-d material (Fig. 2). functionalities does not correspond to the ideal aromatic
For all S-CTF samples a similar loading of sulfur around 1.2 functions within the material, but rather to the expected
to 1.6 wt% has been determined via elemental analysis (Table carbonized units. However, distinction between both func-
1
3
1
). Interestingly, S-CTF-b based on the pyridinic 2,6-DCP tionalizations is rather difficult and even via C-NMR of
20
monomer contains nearly twice the amount of sulfur aer different sulfonated carbons, no clear evidence of the incor-
sulfonation (2.67 wt%, Table 1, entry 6). This might be assigned porated SO H-function could be veried.
to a SO -pyridine adduct formation aer treatment with SO in Thermogravimetric analysis of sulfonated S-CTF-a, -c and -d
3
3
3
the gas-phase. Reproducibility tests conrm this assumption, as indicate the loss of the sulfonic acid functionalities at temper-
ꢁ
the reproduced S-CTF-b contained only 0.72 wt% S, while the atures above 300 C (Fig. S5 and S6†). Decomposition of the
ꢁ
porous material is initiated at temperatures above 400 C. In the
case of CTF-b, desulfonation takes place at somewhat lower
temperatures. This can be attributed to the assumed adduct
formation to the pyridinic function of the 2,6-DCP monomer.
Additionally, CTF-a was sulfonated in the liquid phase to
compare the materials obtained by different procedures (Table S1
and Fig. S7†). Interestingly, the liquid phase sulfonation resulted
in higher sulfur loadings of 3.0 wt% leading to an acidity of
ꢀ
1
0
1
.96 mmol g and thus a sulfonation degree of approximately
2.2%. The amount of acid sites is 2-fold higher compared to the
gas-phase sulfonation method, but can mostly be attributed to
adsorption effects of H SO on the materials surface and adduct
2
4
formation to the triazine moieties, which cannot be suppressed
in the liquid phase. This assumption can further be conrmed by
thermogravimetric analysis as the desulfonation begins at
Fig. 1 TEM images of (a) untreated CTF-d and (b) S-CTF-d after
sulfonation with SO in the gas-phase.
3
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Fig. 2 SEM/EDX mapping images of S-CTF-d. (a) SEM of S-CTF-d particle. (b) C mapping (red). (c) S mapping (blue). The scale bars represent
mm.
1
Table 2 Acidities of sulfonated CTF materials determined via titration,
ICP-OES measurements and elemental analysis to calculate the
degree of sulfonation
Catalytic hydrolysis of cellobiose to glucose
The synthesized S-CTF materials were tested for the hydrolysis
of 10 wt% aqueous cellobiose solution at 150 C. The results of
ꢁ
a
b
c
d
Titration
[mmol g
ICP-OES
EA
S-degree
[%]
the tests are shown in Table 3. All the sulfonated materials were
catalytically active for the hydrolysis. Very high cellobiose
conversions up to 88% are attainable over mesoporous cata-
lysts, whereas only 31–55% were reached over microporous
catalysts for 2 h of reaction (Table 3, entry 5 and 11). Prolon-
gation of the reaction time for another 2 h did not bring
a signicant increase of the conversion (Table 3, entry 6 and 12).
ꢀ
1
ꢀ1
ꢀ1
No. Material
]
[mmol g
]
[mmol g ]
1
2
3
4
5
S-CTF-a
S-CTF-b
S-CTF-b*
S-CTF-c
S-CTF-d
0.38
0.59
—
0.53
0.61
0.36
0.60
—
0.34
0.46
0.37
0.83
0.23
0.40
0.49
4.7
10.7
2.8
5.1
10.1
a
Ion exchange in saturated NaCl solution followed by titration with This points at deactivation of the microporous catalysts under
b
NaOH (0.005 M). Derived from sulfur content determined via ICP-
OES analysis. Calculated from sulfur content determined via
elemental analysis. Degree of sulfonation calculated as amount of
the reaction conditions, probably due to adsorption of
substrate/product in the micropores. Additionally, leaching of
c
d
sulfur (mmol) to amount of monomer (mmol) for entire material sulfur corresponding to 20–40% of its initial content in the
assuming complete and ideal polymerization. With S-CTF-b* as
reproduction of S-CTF-b.
materials was observed (Table 3). Leaching of sulfonic groups
into the aqueous medium led to formation of sulfuric acid
under reaction conditions. As a result, acidication took place
during the experiments, and slightly acidic solutions were ob-
tained aer ltration of the reaction mixtures.
relatively low temperatures in comparison to the S-CTF-
a sulfonated in the gas-phase (Fig. S5†).
Hydrolysis of cellobiose can proceed both homogeneously
It is well-known that sulfonated carbons can catalyze the
and heterogeneously catalyzed, as shown in Fig. 3. The rst
hydrolysis of cellulose and cellobiose to glucose at high conver-
+
37
orders on cellobiose and H active species were assumed. The
parallel processes yield glucose with the rate constants of
hydrolysis khomo and khetero, respectively. Apparently, the overall
20
sions and yields. Since the CTF materials presented in this work
can be allocated to sulfonated N-doped carbon systems, we
decided to investigate them within the same catalytic reaction in
order to verify their successful functionalization.
rate constant for the hydrolysis can be expressed as khydr
¼
a
Table 3 Results of catalytic tests of S-CTF catalysts for hydrolysis of cellobiose
ꢀ1
+
ꢀ1 ꢀ1
hetero [L mol h ]
No.
Catalyst
t [h]
X [%]
Y [%]
pH
Leached sulfur [%]
khydr [h
]
k
homo [H ]/khydr [%]
k
1
2
3
4
5
6
7
8
9
S-CTF-a
S-CTF-a
S-CTF-a
S-CTF-b
S-CTF-b
S-CTF-b
S-CTF-c
S-CTF-c
S-CTF-c
S-CTF-d
S-CTF-d
S-CTF-d
1
2
4
1
2
4
1
2
4
1
2
4
32.9
59.2
88.1
3.4
30.6
34.6
41.3
49.6
85.4
20.0
55.8
66.2
33.3
59.7
85.6
10.9
24.7
27.8
42.7
51.5
83.9
23.5
62.3
66.0
2.5
2.5
2.2
3.3
3.2
3.2
2.7
2.6
2.6
2.7
2.6
2.6
34.0
42.4
48.4
38.1
43.8
29.8
29.5
24.7
46.2
27.8
32.0
32.9
0.58
0.33
0.62
0.59
53
24
44
46
7.2
5.0
8.6
6.5
1
1
1
0
1
2
a
ꢁ
Reaction conditions: 1–4 h, 150 C, 30 bar of N
2
at 750 rpm. Acid : cellobiose molar ratio (given in brackets): S-CTF-a (1.7 mol%), S-CTF-
b (0.7 mol%), S-CTF-c (1.8 mol%) and S-CTF-d (1.4 mol%).
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Fig. 3 Parallel homogeneous and heterogeneous routs for cellobiose hydrolysis in the presence of S-CTF. Rhomo, Rhetero, and Rhydr denote the
ꢀ
1
ꢀ1
rate of homogeneous hydrolysis, the rate of heterogeneous hydrolysis, and the apparent reaction rate (mol L
h
). The rate constants of
ꢀ1
ꢀ1
ꢀ1 ꢀ1
ꢀ1
homogeneous (khomo, L mol
[
h
) and heterogeneous (khetero, L mol
h
) hydrolysis as well as overall rate constant khydr (h ) are considered,
H ] and [cellobiose] denote concentrations (mol L ) of these species in aqueous phase, Ccat is concentration of the catalyst (g L ). [H ]surf is
+
ꢀ1
ꢀ1
+
ꢀ1
a surface concentration of acidic groups (mol g ).
+
+
(k
homo[H ] + khetero[H ]surf
C
cat). The values of khydr were obtained dehydration of glucose under acidic conditions. A generally
from the results of cellobiose hydrolysis in the presence of the accepted mechanism of humin formation comprehends
38
sulfonated CTF materials (Table 3).
condensation of the furanics with saccharides. Apparently,
Homogeneous hydrolysis of cellobiose under the applied adsorption of humins onto CTF is favorable due to high affinity
39
conditions was investigated. A blank test of cellobiose hydro- of HMF towards aromatic functionalities of CTF. Furthermore,
lysis demonstrated that hydrolysis takes place even in the the pH value of the reaction mixture aer the reaction over CTF
absence of a catalyst. The hydrolysis of cellobiose accelerated + H SO (2.9) was slightly higher compared to the pH value of
2 4 0
during the course of the experiment (Fig. S9†). This is in line 2.4. This can be explained by adsorption of sulfuric acid onto
with acidication of the aqueous medium from pH 5.2 till pH 3 basic centers of the CTF material. We performed a control
for the rst hour of the hydrolysis. Further prolongation of the experiment to investigate adsorption of H
2 4
SO onto CTF at
reaction did not lead to dramatic change of the pH value room temperature. Stirring of 1.6 mM H SO with CTF-a led to
2
4
(Fig. S9†). The observed pH change can be explained by total neutralization of the solution, apparently due to interac-
formation of acidic by-products upon dehydration of saccha- tion of the strong acid with nitrogen of triazine rings (Fig. S10†).
rides. It must be noted that the reaction mixture turned The values of khomo were estimated based on the obtained data
brownish upon hydrolysis in pure water. Clearly, despite rather (Table 4). Interestingly, the rate constants appear to be similar
high selectivity for glucose (88–95%), the saccharides partly for different reaction conditions and independent of the nature
2 4 2 4
underwent degradation forming humins. Next, H SO was of acid (H SO or organic acids produced due to dehydration of
tested for hydrolysis of cellobiose. In these tests, the concen- saccharides). The presence of the CTF-a material does not
tration of sulfuric acid (ca. 1.6 mM, pH 2.4) was close to that inuence the rate constant of cellobiose hydrolysis. Data on pH
obtained over S-CTFs due to leaching of sulfur (Table 3). Again, of reaction mixture (Table 3), overall hydrolysis constants khydr
darkening of the reaction solution took place owing to forma- (Table 3), concentration of surface acidic sites (Table 2), and
ꢀ
1
ꢀ1
tion of humins. Finally, we studied catalytic activity of non-
khomo (ca. 123 L mol
h , according to the data from Table 4)
sulfonated CTF-a and a mixture of CTF-a with H SO . Interest- enable estimation of khetero values according to the equations
2
4
ingly, ltration of the reaction mixture aer the experiments presented in Fig. 3. The results of these calculations are listed in
gave transparent solutions. This indicates efficient adsorption Table 3, indicating that the homogeneous reaction makes
of humins onto CTF under the reaction conditions. Furanic a contribution of 24–53% to the apparent rate of cellobiose
compounds, mainly HMF, are products of consecutive hydrolysis. The apparent khetero appeared to be similar for all the
materials, though a somewhat lower value was observed for
CTF-b.
ꢀ
1
Table
4
Estimated rate constants of homogeneous cellobiose
Activation energy of the hydrolysis equals ca. 115 kJ mol in
the presence of the most active catalyst S-CTF-a (Fig. S11†),
similar to the previously obtained data for molecular acids.
hydrolysis
37,40
ꢀ1
ꢀ1
No.
Catalyst
No
k
homo, L mol
h
Importantly, share of the leached out sulfur of ca. 20% does not
dramatically depend on the reaction temperature (Fig. 4). This
indicates the presence of weakly bound sulfur-containing
groups, which are easily removed upon contact with hot
1
2
3
4
134
117
127
114
H
2
SO
CTF-a
SO
4
H
2
4
+ CTF-a
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Fig. 4 (a) Temperature dependency of the cellobiose conversion (X), glucose yield (Y) and sulfur leaching; (b) hot filtration test. Reaction
ꢁ
conditions: 2 h, 110–150 C, 30 bar of N
2
at 750 rpm, 5 mL of 10 wt% cellobiose, 50 mg of S-CTF-a.
cellobiose conversion to 12% occurred in the third run. There-
fore, catalyst deactivation cannot only be attributed to the
leaching of the catalytically active species. Our results on
molecular H SO physically mixed with CTF-a (see above)
2
4
suggest that the materials are very prone to adsorption of partly
dehydrated by-products, which probably brings about blockage
of active sites and drop of catalytic activity.
Further improvements of the catalytic activity can possibly
be achieved by optimization of the procedure of catalyst
41,42
synthesis and pretreatment,
as well as conducting the reac-
tion under continuous conditions to facilitate the removal of
43,44
(by)-products from the reactor.
ꢁ
Fig. 5 Recycling study of S-CTF-a. Reaction conditions: 2 h, 150 C,
0 bar of N at 750 rpm, 5 mL 10 wt% cellobiose, 75 mg S-CTF-a.
3
2
Conclusions
Sulfonation of CTF materials was conducted for the rst time by
water. Previous investigations suggest that sulfonated carbon treating the materials with gas-phase SO . This procedure was
materials also exhibit weakly- and strongly bound sulfonic successfully applied to obtain four S-CTFs which vary in
3
4
1,42
groups.
water results in removal of the weakly bound sulfur, whereas of 5–10% and an acid surface concentration of around 0.4–
Pre-treatment of the sulfonated carbons with hot composition of the monomers. Sulfonation degrees in the range
ꢀ1
strongly bound groups were stable under the reaction condi- 0.8 mmol g were reached. The acid sites are evenly distributed
tions of cellobiose hydrolysis.
41,43
This was also indicated via within the CTF materials, while the material itself does not
a hot ltration test resulting in only slight increase of conver- show any signs of decomposition aer sulfonation. Thermal
sion and yield aer ltration of the solid catalyst and processing stability of the sulfonated S-CTFs suggests a covalent binding of
the liquid phase for additional 2 hours (Fig. 4b).
the introduced acid sites. Notably, the CTFs preserve their
We studied the recycling of the S-CTF-a catalyst (Fig. 5). As structural properties and slight losses in specic surface area as
expected, high amounts of sulfur, corresponding to 43% well as porosity can be assigned mostly to the space demanding
content in solid catalyst, were only leached in the rst run. In acid sites.
the following two runs, only 4 and 2% of the initial S concen-
Catalytic testing provides evidence of the successful acid
tration were detected in the product solution. In total, ca. 49% functionalization of the polymers. The S-CTF materials were
of the initial sulfur content was leached out of the catalyst active when applied in the catalytic hydrolysis of cellobiose to
during the three consecutive runs, which was conrmed by ICP- glucose yielding up to 86% glucose in 4 hours at a rather high
OES analysis of the liquid aer reaction and the elemental substrate-to-catalyst weight ratio of 10. Very high selectivity of
analysis of the used catalyst. A signicant drop of the cellobiose glucose formation over 90% was observed for hydrolysis in the
conversion from 78 to 36% was observed for the rst and the presence of S-CTF. Leaching of sulfur took place during the
second run, which can be explained by considerable loss of course of hydrolysis, as it is expected for any sulfonated mate-
sulfur during the rst run. Despite very low leaching of sulfur rials under the hydrothermal reaction conditions. Additionally,
during the second and the third runs, further decrease of the co-formation of acidic by-products was observed. Ca. 24–53% of
cellobiose were hydrolyzed over S-CTFs due to contribution of
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Acknowledgements
We acknowledge nancial support from the DFG (PA1689/1–1)
and funding from the Excellence Initiative of the German
federal and state governments. SEM/EDX mapping and TEM
imaging was performed at the Center for Chemical Polymer
Technology CPT, which is supported by the EU and the federal
state of North Rhine-Westphalia (grant no. EFRE 30 00 883 02).
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