Mechanism for the formation and growth of carbonaceous spheres from sucrose by hydrothermal carbonization

Article abstract of DOI:10.1039/c5ra26725k

We report a new three-step mechanism for the formation and growth of carbonaceous spheres by hydrothermal carbonization of saccharides using sucrose as a precursor material. Carbonaceous spheres with small diameters and narrow size distribution were synthesized via a rapid heating route, and a notable phenomenon of a sudden drop in the mean diameter of the carbonaceous spheres at low concentration with the extension of time was observed. The morphology, chemical structure of carbonaceous spheres and the chemical composition of residual solutions were analysed by field emission scanning electron microscope (FESEM), Fourier transform infrared spectroscopy (FT-IR) and solution ¹³C nuclear magnetic resonance (NMR) respectively. Based on these results, evolution of solid products is clearly revealed. The formation contains two stages, and oversaturation of primary particles attributed to autocatalysis of fructose by the yielded acid (formic acid) results in the appearance of large amounts of carbonaceous spheres in the second stage of formation, which accounts for the sudden drop in mean diameter.

Full text of DOI:10.1039/c5ra26725k

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Mechanism for formation and growth of carbonaceous spheres
from sucrose by hydrothermal carbonization
†
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
ab
Yujie Qi, Mu Zhang, Lin Qi and Yang Qi *
DOI: 10.1039/x0xx00000x
We report a new three-step mechanism for the formation and growth of carbonaceous spheres by hydrothermal
carbonization of saccharides using sucrose as precursor material. Carbonaceous spheres with small diameter and narrow
size distribution were synthesized via rapid heating route, and a notable phenomenon of sudden drop in mean diameter of
carbonaceous spheres at low concentration with the extension of time was observed. The morphology, chemical structure
of carbonaceous spheres and the chemical composition of residual solutions were analysed by field emission scanning
www.rsc.org/
1
3
electron microscope (FESEM), Fourier transform infrared spectroscopy (FT-IR) and solution C nuclear magnetic resonance
(NMR) respectively. Based on these results, evolution of solid products is clearly revealed. The formation contains two
stages, and oversaturation of primary particles attributed to autocatalysis of fructose by the yielded acid (formic acid)
results in appearance of large amounts of carbonaceous spheres in the second stage of formation, which accounts for
sudden drop in mean diameter.
1
1,12
13
or catalyst support materials. Especially for the
carbons,
Introduction
carbonaceous spheres with uniform size, which could be
served as sacrificial templates for fabricating hollow spheres of
In recent years, hydrothermal carbonization (HTC) has
attracted great attention due to the convention of biomass to
2 3 2 2 5
inorganic materials (Ga O , GaN, ZnO, SnO , Ta O , etc.),
1
,2
applied widely in the fields of photocatalysis, Dye-Sensitized
4-18
Solar Cell and gas sensor.
outstanding carbonaceous materials. However, this process
involved with saccharides was initially carried out to study the
1
1
Apart from polymers, carbohydrates, such as xylose,
9
20
22
3
,4
mechanism of natural coalification in the early 20th century.
1
1,12,21-23
20,24
11,22
glucose,
fructose,
sucrose,
starch,
Later on, 5-hydroxymethylfurfural (HMF), levulinic acid, lactic
acid and furfural as valuable chemicals generated in
hydrothermal carbonization of biomass were researched
extensively to improve their yields with assistance of
supercritical waters, organic solvents, and catalysts, besides,
detailed chemical routes and reaction kinetics for the
conversion of biomass to HMF were also investigated
1
1,25
26
and cyclodextrins, have been frequently used
cellulose,
as raw materials to synthesize carbonaceous spheres by HTC
process. Although these particles synthesized are in the size
range of 0.2-10 μm, carbonaceous spheres with uniform size
are difficult to be found among these cases. The minimum
mean sizes of carbon spheres obtained from different
carbohydrates varied widely in the previous literatures, viz.
5
-8
simultaneously.
Nevertheless, the solid residues from
2
1
20
1
50 nm for glucose, 170 nm for xylose, 290 nm for
2
hydrothermal carbonization of biomass were once considered
as useless by-products. The carbonaceous spheres with regular
0
fructose. Besides, detailed variation tendencies and size
,10 distributions of hydrothermal products are lacking.
9
shape firstly used as anode materials for lithium storage,
There are two main different mechanisms about formation
proposed in literatures of hydrothermal carbonization of
carbohydrates: the former one based on Fourier transform infrared
spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and
Raman characterizations, considers that the stable oxygen groups
which triggered further synthesis of functional carbonaceous
materials for various purposes. Up to date, carbonaceous
materials with regular spherical shape and controllable size are
promising materials as precursors for activated spherical
2
2,25
such as ether or pyrone constitute inner core,
and that
formation and growth of carbonaceous spheres follow the LaMer
a.
Institute of Materials Physics and Chemistry, School of Materials Science and
27
13
model; the latter one based on solid state C nuclear magnetic
Engineering, Northeastern University, Shenyang 110819, P. R. China. E-mail:
qiyang@imp.neu.edu.cn
Key Laboratory for Anisotropy and Texture of Materials, School of Materials
Science and Engineering, Northeastern University, Shenyang 110819, P. R. China.
Electronic Supplementary Information (ESI) available: Two tables showing
resonance (NMR) results, emphasizes on the furans rather than
b.
28,29
benzene rings in the scaffold of carbonaceous spheres.
As
1
3
single-quantum double-quantum (SQ-DQ)
C
correlation
†
synthesis conditions and statistic results, and two figures supplying detailed
evidences for proposed mechanism. See DOI: 10.1039/x0xx00000x
experiments provided possible ways to study the local structure of
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Fig. 1 SEM images and diameter histograms of carbonaceous spheres under various experimental conditions. Sucrose hydrothermally treated (180 ℃) at 0.333 M in Slow Heating
route for (a) 2.5h, (b) 2.75h, (c) 3h, (d) 3.5h, in Rapid Heating route for (e) 2.5h, (f) 2.75h, (g) 3h, (h) 3.5h
.
theory that furanic rings derived from polymerisation of HMF are
scaffolds of carbonaceous spheres is convincing in analysing
chemical structure of carbonaceous spheres.
In the present work, experiments were conducted under
hydrothermal conditions using sucrose as precursor to
elucidate accurate processes involved with formation and
growth. Based on results, a new three-step mechanism for
formation and growth was proposed. The formation of
carbonaceous spheres consists of two distinct stages, mainly
due to significant difference in yield, and the growth can be
divided into three stages owing to varieties of growth units.
Experimental procedures
Materials
Fig. 2 The variation of diameters of carbonaceous spheres obtained at 0.333 M for All the chemicals were of analytical grade purchased from
different reaction times in Slow Heating and Rapid Heating routes. The bound of error Sinopharm Chemical Reagent Co., Ltd. and were used without
bar represents 90% range of statistical samples, and the intermediate point reflects the
further purification.
mean diameter of carbonaceous spheres among 50% range in each group.
Synthesis of carbonaceous spheres
3
0
The typical hydrothermal carbonization of sucrose was carried
out according to the following procedure. Firstly, 0.01 mol
carbonaceous spheres and probe direct carbon-carbon bonds, the
2
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Fig. 3 SEM images and diameter histograms of carbonaceous spheres under various
experimental conditions. Sucrose hydrothermally treated in Slow Heating route for 4h
at (a) 0.100 M, (b) 0.167 M, (c) 0.233 M, (d) 0.300 M, (e) 0.333 M.
Fig. 4 SEM images and diameter histograms of carbonaceous spheres under various
experimental conditions. Sucrose hydrothermally treated in Rapid Heating route for 4h
at (a) 0.067 M, (b) 0.100 M, (c) 0.167 M, (d) 0.233 M, (e) 0.300 M, (f) 0.333 M.
Two different heating routes were applied in hydrothermal
process. The Slow Heating (SH) represented a heating process
from ambient temperature to 180
℃ with the rise velocity of
-1
temperature maintaining 5
℃
initiated once temperature reached 180
·min , in which reaction time
. Moreover, the
℃
reaction time in the Rapid Heating (RH) route initiated once
reactor vessel was directly transferred into muffle furnace
maintaining 180 ℃. Herein, 0.067, 0.100, 0.167, 0.233, 0.300,
.333 M sucrose solutions were selected for hydrothermal
0
carbonization. The detailed experimental parameters applied
here are listed in Table S1 and Table S2.†
Characterization
The morphologies and sizes of solid products were
characterized by a field emission scanning electron microscope
(FESEM, ZEISS & ULTRA PLUS). The diameters of carbonaceous
Fig. 5 The variations of diameters of carbonaceous spheres obtained in Slow Heating spheres were measured by image processing software from
and Rapid Heating routes with different concentrations. The bound of error bar
SEM images of each sample, and statistical analysis was
represents 90% range of statistical samples, and the intermediate point reflects the
performed subsequently. The sampling number of each group
mean diameter of carbonaceous spheres among 50% range in each group.
was more than 1000. The fourier transform infrared
spectroscopy (FT-IR) spectra were measured by a VERTEX
sucrose was dispersed in 30 ml water, stirred vigorously for 20
7
0 (Bruker) spectrometer with KBr pellet technique ranging
13
min to form clear and transparent solution. The solution was
then transferred to a 45 ml Teflon-lined stainless steel
-1
2
from 500 to 4000 cm . For C NMR analysis, D O was added
into 0.1 M sucrose solutions reacted for various reaction times.
13
The C NMR spectra of solutions were recorded using Bruker
autoclave, heated up to 180
℃
and maintained for a period of
time. After this, the reactor vessel was immediately immersed
in water to quench the reaction. Finally, the precipitates were
collected by centrifugation (4000 rpm, 30 min), and rinsed with
distilled water and anhydrous ethanol more than three times,
Avance 600 MHz (14.09 T) spectrometer.
Results and discussion
then dried at 80
℃ in a vacuum furnace for 12 h.
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Fig. 6 SEM images and diameter histograms of carbonaceous spheres under different experimental conditions. Sucrose hydrothermally treated in Rapid Heating route at 0.100 M
for (a) 3h, (b) 3.5h, (c) 3.75h, (d) 4h, (e) 4.5h, (f) 5h, (g) 5.5h.
Fig. 7 SEM images and diameter histograms of carbonaceous spheres under different experimental conditions. Sucrose hydrothermally treated in Rapid Heating route at 0.067 M
for (a) 3h, (b)3.5h, (c) 3.75h,(d) 4h, (e) 4.5h, (f) 5h, (g) 5.5h.
4
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Fig.
8
The variations of diameters of carbonaceous spheres obtained at low
concentration for different reaction time in Rapid Heating mode. The bound of error
bar represents 90% range of statistical samples, and the intermediate point reflects the
mean diameter of carbonaceous spheres among 50% range in each group.
Variation tendency of products
Reaction temperature (180
℃) was regarded as constant
parameter in carbonization process. Initially, a series of
experiments were conducted at 0.333 M for different reaction
times in both Slow Heating (SH) and Rapid Heating (RH) routes.
The results demonstrate that no solid product could be
obtained when reaction time was less than 2.5 h in SH route
and 2 h in RH route, only brown and viscous liquids existed.
Besides, the mean diameters of carbonaceous spheres
obtained in both heating routes are no less than 200 nm (Table
S1,† Fig. 1 and Fig. 2). Although carbonaceous spheres
appeared in RH route for 2 h and 2.25 h, the statistical results
were unreliable due to low yield. There is no remarkable
change between SH and RH routes in mean diameter, range
and standard deviation of products under high concentration.
Furthermore, sucrose solutions with different concentrations
were hydrothermally treated in both SH and RH routes for 4 h,
as listed in Table S2.† SEM images and distribuꢀon histograms
of carbonaceous spheres are presented in Fig. 3 and Fig. 4. It is
clear that carbonaceous spheres with regular spherical shapes
and smooth surfaces were obtained, and that the mean
diameters of carbonaceous spheres reduced and the size
distribution narrowed with decreasing sucrose concentrations
in both heating routes. It is worth noting that no solid products
appeared in the case of 0.067 M solution reacted for 4 h in SH
route. Adhesions between carbonaceous spheres could be
observed in Fig. 4(f), indicates that the growth of products is
quicker in RH route than that in SH route. Meanwhile, the
bound of statistic range and mean diameter in RH route
exceed those in SH route when concentration was beyond
Fig. 9 The FT-IR spectra of typical products obtained from (a) Slow Heating and (b)
Rapid Heating routes.
In order to elucidate accurate processes involved with
formation and growth of carbonaceous spheres with small
diameter and narrow size distribution, 0.067 and 0.100 M
solutions of sucrose were hydrothermally reacted in RH route
for various reaction times. Fig. 6 presents SEM images and
diameter histograms of products obtained at 0.100 M
solutions in RH route. The inspiring results show that the mean
diameters of most groups are below 200 nm. Statistic results
show that the mean diameters and standard deviations of
products obtained at 0.100 M for 3 h, 3.5 h, 3.75 h, 4 h, 4.5 h,
5
3
±
h, 5.5 h in RH heating route are 162.22 ± 50.32, 189.78 ±
5.38, 138.43 ± 22.82, 192.81 ± 38.70, 192.38 ± 31.58, 208.28
37.99, 288.08 ± 46.09 nm respectively, as listed in Table S2.†
That is to say, there is a sudden drop in mean diameter and a
narrow size distribution in the case of 0.100 M for 3.75 h in RH
route, which is obviously presented in Fig. 6 and Fig. 8. Similar
phenomenon that mean diameter of products decreases with
reaction time appears in the cases of 0.067 M. From 3 h to 3.5
h the mean diameter increases mainly due to growth of
colloidal spheres, after 3.5 h, the mean diameter and bound of
0.167 M, on the contrary, the values in RH route are lower
than values in SH route when concentration was below 0.167
M, as shown in Fig. 5. So, a brief conclusion could be reached
that Rapid Heating route is more suitable for synthesizing
carbonaceous spheres with smaller sizes.
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3
13
Fig. 10 (a) The C solution NMR spectra of 0.100 M sucrose solutions treated for different reaction times at 180 ℃, (b) typical C solution NMR spectrum of isomers of glucose and
fructose from solution reacted for 2h.
-
1
20,22,25
9
0% range of carbonaceous spheres decrease gradually, as 1510 cm , are attributed to C=C vibrations.
There are
-1
shown in Fig. 7 and Fig. 8. In fact, the mean diameter of whole also several characteristic absorption bands at 3420 cm (O-H
-1
products reduce significantly due to newly-formed stretching vibration), 2926 and 1461 cm (stretching vibration
-1
carbonaceous spheres in large quantities, whose sizes are of aliphatic C-H), 1023 cm (characteristic furan 1030 to 1015
-1
relatively small. This phenomenon is related to the formation cm band), whereas the bands at 1708 and 1384 cm
and growth of carbonaceous spheres, and a detailed correspond to stretching vibrations of C=O (carbonyl, or
-1
1
2,20,22,23
illustration is available in following sections.
carboxyl) and C-H in the carbonyl, respectively.
-1
Moreover, the bands at 1299 and 1213 cm are attributed to
-1
C-O-C stretching vibration, and band at 796 cm is assigned
to out-of-plane bending vibrations of C-H in RCH=CHR groups.
2
0
Chemical Characterization
FT-IR spectra of the typical products obtained in SH and RH
routes are shown in Fig. 9. It is obvious that there is no
significant difference among these spectra, confirming that
heating route and reaction time have little influence on the
species of functional groups. The bands positioned at 1615 and
1
3
Fig. 10(a) clearly shows solution C NMR spectra of 0.100 M
sucrose solutions treated for various reaction times at 180
Sucrose (103.54, 92.05, 81.27, 76.38, 73.97, 72.59, 72.30,
℃
.
7
1.05, 69.19, 62.33, 61.30, 60.09 ppm) remained stable in 1h,
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3
1,32
from
D
-fructose and
D
-glucose to HMF.
As a consequence,
there is a steady flow of HMF in initial stage.
In the second step, polycondensation reactions occur
between dissolvable HMF molecules, in similar mechanism
2
8
proposed by Zhang et al. Three most possible cross-linking
pathways among HMF molecules, including two forms of α-
carbon bindings and one form of β-carbon binding have been
1
3
29
confirmed based on C DQ-SQ MAS NMR results. With the
extension of time, the process of cross-linking proceeds to
form three dimensional furanic structure. This structure grows
by continuous dehydration of HMF monomer, and the relative
quantities of dissolvable functional groups (i.e., hydroxy and
aldehyde groups) decrease with consistent increase in volume,
which lead them to become hydrophobic and precipitate from
solution eventually. The nano-sized precipitations, named as
2
8
“
primary particles”, which are typically around 5 nm in size,
Fig. 11 The approximate durations of chemicals existed in 0.100 M sucrose solutions.
are also found in our solid products (0.100 M for 3.5 h, 0.067
M for 3 h and 3.5 h, RH route) after centrifugation, as shown in
Fig .S1.†
while no chemical shift of sucrose could be found in solution
In the third step, formation and growth of small-sized
1
3
C NMR spectrum for 2h, indicating that sucrose totally carbonaceous spheres are involved. Herein, carbonaceous
hydrolyzed into fructose and glucose. The isomers of glucose spheres obtained from almost all cases could be classified as
and fructose, including α- -glucopyranose (92.05 72.78
two batches according to differentiation of the formation time.
1.49 71.34 69.62 60.56 ppm), β-
-glucopyranose The primary particles aforementioned are inclined to
95.87 75.85 75.77
fructofuranose (104.37
D
，
，
7
，
，
，
D
(
，
，
，
74.16
，
69.58
，
，
60.73 ppm), α-
D
-
aggregate due to diminution of surface free energy via
reactions among functional groups existing in their surfaces. As
a consequence, the first batch of carbonaceous spheres
，
81.99，
81.21
76.02 62.92
，
，
，
6
1.06 ppm), β-
D
-fructopyranose (98.01
63.26 ppm) and β-
75.36 74.41 62.66
，
69.68， 69.18
6
7.55
，
63.88
，
D
-fructofuranose (101.43 appears in hydrothermal solution in spite of relatively small
3
1,32
，
80.57
，
，
，
，
62.34 ppm),
were amount of primary particles in early stage (carbonaceous
detected, as shown in Fig. 10(b). 5-Hydroxymethylfurfural spheres obtained at 0.100 M for 3 h in RH route). Formic acid
HMF) (180.37, 161.28, 151.69, 126.82, 110.89, 56.04 ppm) in was initially detected in 0.100 M solution for 3 h by solution
(
1
3
a trace amount was also found in solution reacted for 2 h.
C NMR, besides, numerous primary particles were only found
Besides, formic acid (166.20 ppm) was initially detected in 3 h in 0.100 M solution reacted for 3.5 h. In other words, formic
solution, while both acetic acid (176.69, 20.40 ppm) and acid produced acts as catalyst to accelerate conversion of
levulinic acid (177.43, 37.66, 29.23, 27.77 ppm) for 3.75 h fructose to HMF, which is consistent with autocatalysis process
3
4,35
solution, respectively. Chemical structures of these four of fructose,
substances are schematically shown above the NMR spectra. concentration in 0.100 M solutions after 3 h. Simultaneously,
And the approximate durations of chemicals existed in 0.100 the amount of newly-formed primary particles grows
and leads to rapid increase of HMF
M sucrose solutions are shown in Fig. 11.
dramatically within the duration from 3 h to 3.5 h in 0.100 M
solutions. From 3.5 h to 3.75 h, the amount of newly-formed
primary particles reaches a oversaturated state, and they
aggregate rapidly to form the second batch of carbonaceous
spheres in massive amount. In general, the precipitation of
primary particles is dominant in the period from 3 h to 3.5 h,
the growth of first batch of carbonaceous spheres as
supplementary; the period from 3.5 h to 3.75 h is dominated
by the formation of second batch of carbonaceous spheres,
accompanied by the growth of both batches of carbonaceous
spheres. The size of second batch of carbonaceous spheres is
smaller than that of first batch due to lacking of enough
growth, and the quantity of second batch is much more than
first batch, these make a rational interpretation of the sudden
drop in mean diameter and narrow size distribution of
carbonaceous spheres in 0.100 M solution reacted for 3.75 h.
The whole growth of carbonaceous spheres can be divided
into three stages based on the experimental results and
varieties of growth units. In the first growth stage, both
Mechanism for formation and growth of carbonaceous spheres
The experimental results presented above demonstrate that
whole formation process of carbonaceous spheres is complex,
and it includes several by-products. The proposed mechanism
is schematically illustrated in Fig. 12. In the first step, the
formation of HMF molecules holds dominant position. At first,
sucrose hydrolyzes into fructose and glucose, catalyzed by
3
3
hydronium ions generated by water autoionization. The
process involved formation of HMF molecules could be divided
into three parts as follows: α-
fructofuranose convert to HMF via intramolecular dehydration
by losing 3H O directly, at the same time, α- -glucopyranose
and β- -glucopyranose transform into β- -fructopyranose, and
β- -fructopyranose transforms into α- -fructofuranose and β-
-fructofuranose by tautomerization, respectively. The
D-fructofuranose and β-D-
2
D
D
D
D
D
D
processes above match well with pathways of conversions
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Fig. 12 (a) Detailed illustration of formation and convention of involved chemicals, (b) mechanism for formation and growth of carbonaceous spheres form sucrose by
hydrothermal carbonization .
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primary particles and HMF molecules can constitute new accelerate dehydration of HMF in initial stage, but also as
surfaces of carbonaceous spheres in first batch via dehydration embedded units in the third growth stage.
reaction. At the early phase of the first growth stage, the
The formation and growth of carbonaceous spheres is not
amount of carbonaceous spheres has no obvious change, the consistent with the conventional LaMer model, which includes
mean diameter of carbonaceous spheres increases slightly outburst nucleation and subsequent growth by molecular
2
7
because of scarcity of growth units, just as the change in the attachment. The LaMer model is heavily dependent on the
range of 3 h to 3.5 h in 0.100 M solution (RH route) shown in concentration of feedstock, and oversaturation is only
Fig. 8. The later phase of the first growth stage is occupied by criterion for nucleation in solution. As mentioned above, the
rapid growth, the quantity of carbonaceous spheres is much second batch of carbonaceous spheres formed due to the
more than that at early phase, and the mean diameter oversaturation of primary particles, and HMF molecules play
increases from 138 to 190 nm within quarter hour, an important role in growth. So no accurate feedstock could be
corresponding to range from 3.75 h to 4 h in 0.100 M distinguished in whole process of formation and growth,
solutions. Here, the residual primary particles which are not neither HMF nor primary particle. Carbonaceous spheres could
involved in formation of second batch of carbonaceous also be obtained in very low concentration (0.050 M for 6 h,
spheres make an enormous contribution to rapid growth of 0.033 M for 10 h, Fig .S2),† which indicates that no standard
whole products by direct depositions on their surfaces in concentration for nucleation and that enough reaction time
abundance. Whereafter, carbonaceous spheres grow slowly in guarantees to generate carbonaceous spheres even in tiny
the second growth stage, which corresponds to the duration concentration of reactant.
from 4 h to 5 h in 0.100 M solutions. No primary particle could
be found in solutions hydrothermally treated for more than batch changes with initial concentration of sucrose, as shown
.5 h in 0.100 M solutions (RH route), which means HMF in Fig. 8. The values are about 0.25 h and 1.5 h for 0.100 and
molecules act as main building units in second growth stage. 0.067 M solutions (0.100 M solution from 3.5 h to 3.75 h,
The carbonaceous spheres already existed provide 0.067 M solution from 3.5 h to 5 h), respectively, while the
The duration of formation of carbonaceous spheres in second
3
a
widespread area of reaction interface for dehydration of HMF mean diameters of products at the end of duration are around
molecules in the second growth stage, so the formation of 138 and 123 nm for 0.100 and 0.067 M. These indicate that the
carbonaceous spheres almost comes to the end. As for the initial concentration of sucrose greatly influences the amounts
third growth stage, the mean diameters of carbonaceous of HMF molecules and primary particles, resulting in changes
spheres increase rapidly after 5 h in 0.100 M groups. The of duration of formation and size of carbonaceous spheres on
porous surface layers of carbonaceous spheres reacted for 5 h the whole. So it is reasonable to assume that carbonaceous
in 0.100 M solution provide sufficient space for embedability spheres with smaller sizes appear in lower concentration and
2
9
of levulinic acid and other molecules, accompanied by longer reaction time, and no distinct reduction in sizes with
dehydration of HMF molecules, which lead to dramatically the time in high concentration as short duration of formation
increase the size of products. Although the accurate in second batch. As a consequence, it is certainly preferable to
boundaries of these three growth stages are difficult to choose carbonaceous spheres reacted at the end of duration
distinguish, fundamental distinctions among them are the of formation of second batch, which have distinct advantages
varieties of growth units.
of small size, narrow distribution and high yield.
Discussion
Conclusions
Based on FT-IR results, cross-link furanic rings derived from
polycondensation of HMF molecules are confirmed by the In summary, the mechanism of formation and growth of
-1
characteristic furan band at 1023 cm . The simplified model is carbonaceous spheres derived from sucrose under
clear that the carbonaceous spheres consist of two parts, inner hydrothermal conditions has been investigated in this study,
core is mostly full of stable cross-linked groups, outer shell and a new model for the formation and growth has been
contains lots of reactive groups (i.e. hydroxyl, carbonyl), which proposed here. The results demonstrate that the carbonaceous
make inner core hydrophobic and outer shell hydrophilic. And spheres formed can be classified as two batches, the first batch
mechanism involved with growth discussed above is consistent of carbonaceous spheres in small quantity comes from
with this model.
There are several acids produced during hydrothermal polycondensation of HMF molecules, and the second batch in
process. The formic acid and levulinic acid are directly large quantity attributes to autocatalysis of fructose by the
aggregation
of
primary
particles
derived
from
5
generated by rehydration of HMF, and acetic acid comes from yielded acid (formic acid) from rehydration of HMF, which lead
fragmentation of fructose. These acids lower the pH value of to the peculiar phenomenon of sudden drop in mean diameter.
solutions, and facilitate the conversion of fructose to HMF.
The growth of carbonaceous spheres should be divided into
And embedded levulinic acid is confirmed in carbonaceous three stages due to varieties of growth units, HMF and primary
3
6
8
1
spheres based on solid state C NMR characterization,
3
29
particles for the first stage, only HMF for the second stage,
levulinic acid is considered indispensable in the growth of HMF and embedded molecules for third stage. Combined with
carbonaceous spheres. So acids can act not only as catalysts to changes of quantity of carbonaceous spheres, the different
growth units contribute to variation of growth rate of solid
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H. M. Abdelaal, E. Pfeifer, C. Grünberg and B. Harbrecht,
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sucrose, due to unavailability of accurate feedstock. In addition,
monodisperse carbonaceous spheres reacted at the end of
duration of formation of second batch are optimal choices
among various cases, due to small size, narrow distribution and
high yield.
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