Carbon Materials as Phase-Transfer Promoters for Obtaining 5-Hydroxymethylfurfural from Cellulose in a Biphasic System

Article abstract of DOI:10.1002/cssc.201901264

Different carbonaceous materials were tested as mass-transfer promoters for increasing the yield of 5-hydroxymethylfurfural (5-HMF) in biphasic cellulose hydrolysis. The benefits of working with a biphasic system (water/methyl isobutyl ketone) under soft acid conditions were taken as starting point (no humins or levulinic acid production), with slow extraction kinetics as the weakest point of this approach. Carbon nanotubes (CNTs) and activated carbon (AC) were proposed to improve 5-HMF liquid–liquid mass transfer. A kinetic analysis of the extraction process indicated the competition between 5-HMF and glucose adsorption as the main cause of the poor results obtained with AC. In contrast, very promising results were obtained with CNTs, mainly at 1.5 wt % loading, with complete transfer of HMF and a high global mass-transfer coefficient. The use of CNTs improved the amount of 5-HMF in the organic phase by more than 270 %.

Full text of DOI:10.1002/cssc.201901264

DOI: 10.1002/cssc.201901264
Full Papers
Carbon Materials as Phase-Transfer Promoters for
Obtaining 5-Hydroxymethylfurfural from Cellulose in a
Biphasic System
Laura Faba, Diego Garcꢀs, Eva Dꢁaz, and Salvador OrdꢂÇez*[a]
Different carbonaceous materials were tested as mass-transfer
promoters for increasing the yield of 5-hydroxymethylfurfural
(5-HMF) in biphasic cellulose hydrolysis. The benefits of work-
ing with a biphasic system (water/methyl isobutyl ketone)
under soft acid conditions were taken as starting point (no
humins or levulinic acid production), with slow extraction ki-
netics as the weakest point of this approach. Carbon nano-
tubes (CNTs) and activated carbon (AC) were proposed to im-
prove 5-HMF liquid–liquid mass transfer. A kinetic analysis of
the extraction process indicated the competition between 5-
HMF and glucose adsorption as the main cause of the poor re-
sults obtained with AC. In contrast, very promising results
were obtained with CNTs, mainly at 1.5 wt% loading, with
complete transfer of HMF and a high global mass-transfer coef-
ficient. The use of CNTs improved the amount of 5-HMF in the
organic phase by more than 270%.
Introduction
The increasing demand for chemicals and drop-in fuels pro-
duced from renewable resources has boosted the interest in
biomass upgrading.[1] Cellulose is the main polymeric constitu-
ent of lignocellulosic biomass, with a highly crystalline polymer
structure, consisting of thousands of d-glucose molecules.[2]
This polymer can be hydrolyzed to glucose sugars, which can
be transformed to a huge variety of valuable chemicals, gener-
ally known as bio-platform molecules.[3] 5-Hydroxymethylfurfu-
ral (5-HMF) is considered to be among the most relevant ones,
with many interesting valorization routes as building block for
the production of polymers (biopolyester building blocks), fuel
additives (2,5-dimethylfuran, 5-ethoxymethylfurfural, ethyl levu-
linate, etc.), and even drop-in fuels after condensation and hy-
drodeoxygenations steps.[4]
of heterogeneous catalysts despite the well-known disadvan-
tages of homogeneous catalysts. In fact, most of the previous
works proposing solid catalysts report the need of expensive
pretreatments of cellulose[8] and the challenge of catalyst foul-
ing caused by humins.[9] Because of these reasons, homogene-
ous catalysis is nowadays considered as technically more
viable, and current works are focused on process optimization
by minimizing the use of mineral acids and reducing the reac-
tion temperature.
Once the glucose is obtained, it has to undergo a further de-
hydration under acidic conditions, which removes three water
molecules and produces 5-HMF. This reaction must be per-
formed at softer conditions than the previous hydrolysis to
prevent side reactions, such as the decomposition into levulin-
ic and formic acid as well as different glucose and 5-HMF poly-
merization routes, yielding insoluble and valueless humins.[10]
However, these soft conditions also limit the productivity by
requiring longer reaction times. Several authors have proposed
a previous isomerization of glucose into fructose, reducing
humin formation (because most of these undesired products
are obtained by oligomerization and side reactions of unreact-
ed glucose) and allowing higher temperatures, resulting in an
increased reaction rate.[11,12] This isomerization requires the co-
presence of a Brønsted acid catalyst (homogeneous; HCl) and
a Lewis acid (heterogeneous; e.g., acid zeolites or mesoporous
silicates). The production of 5-HMF is strongly enhanced by
this route,[13] which lowers the activation energy of the dehy-
dration step by more than 30%, but adsorption usually de-
creases the carbon balance.[11]
5-HMF production from cellulose requires two main steps, as
summarized in Figure 1: (1) hydrolysis of cellulose towards glu-
cose; and (2) glucose dehydration to produce 5-HMF. The hy-
drolysis can be enzymatically catalyzed.[5] However, the high
sensitivity of these technologies, as well as their high cost,
limit their industrial implementation. Therefore, chemical hy-
drolysis is nowadays in the spotlight as a promising alternative
to the enzymatic route. Severe conditions in terms of pressure
and temperature as well as the use of mineral acids such as
HCl and H2SO4 with Brønsted acid sites are widespread.[6,7] Cel-
lulose is an insoluble polymer, which hinders the effective use
[a] Dr. L. Faba, D. Garcꢀs, Dr. E. Dꢁaz, Prof. Dr. S. OrdꢂÇez
CRC Research Group
Department of Chemical and Environmental Engineering
University of Oviedo
c/ Juliꢃn Claverꢁa s/n, 33006, Oviedo (Spain)
E-mail: sordonez@uniovi.es
If the whole process, from cellulose to 5-HMF, was per-
formed in a one-pot configuration, the mechanism and mass-
transfer limitations would be difficult to control. Thus, this pro-
cess is mainly studied considering mineral acids (typically HCl).
In this case, side products (acids and humins) cannot be pre-
The ORCID Identification number(s) for the author(s) of this article can
be found under:
https://doi.org/10.1002/cssc.201901264.
ChemSusChem 2019, 12, 1 – 10
1
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Figure 1. Reaction pathway for cellulose conversion into 5-HMF.
vented because experiments are typically conditioned by the
pressure and temperature required in the first step.[10a,14,15] To
overcome this drawback, various ionic liquids and polar aprotic
organic solvents have been proposed.[16] In all cases, results
were limited by the slow kinetics if hydrolysis was performed
in any non-aqueous solvent. With these premises, biphasic sys-
tems have been proposed,[17] based on the hydrophilic charac-
ter of all compounds involved, except 5-HMF. Under ideal con-
ditions, the continuous extraction of 5-HMF to an organic
phase prevents its further degradation to levulinic acid or
humin polymers in addition to minimizing the need for subse-
quent purification steps. However, the choice of solvent is a
key parameter because the extraction kinetics must be fast
enough to prevent the reaction from continuing in the aque-
ous phase with the 5-HMF that has not been efficiently extract-
ed.
reactions, they have not been studied for promoting cellulose
upgrading.
The aim of this work is to study the role of carbonaceous
materials as mass-transfer promoters in the context of the se-
lective transformation of cellulose into 5-HMF in a biphasic
system. In addition to the use of these mass-transfer promot-
ers, the system methyl isobutyl ketone (MIBK)/water applied to
the cellulose transformation without any pretreatment and at
these conditions is also a novelty in the literature because pre-
vious works propose the use of microwaves and ultrasonica-
tion to pretreat the cellulose[21] or use complex catalytic and
solvent systems.[22,23] MIBK is a good alternative from the point
of view of the sustainability of the process, considering its
green origin, its large availability, the high solubility of 5-HMF
in this solvent, and the low toxicity and boiling point in com-
parison to other alternatives tested for biphasic systems, such
as THF, acetonitrile, or DMSO.[21a,22] Two different types of car-
bons were tested as mass-transfer agents: active carbon (AC)
and carbon nanotubes (CNTs). The amount of material required
was optimized with the best one (CNTs), and results are ex-
plained in terms of extraction kinetics and equilibrium. The
motivations for selecting these materials were the high adsorp-
tion capacity of AC and the regular structure and weak surface
functionalization of CNTs. Other carbonaceous materials (acti-
vated graphite or carbon nanofibers) are supposed to have in-
termediate behaviors.
Mass-transfer limitations are the weakest point of these mul-
tiphasic reactions. Several works have dealt with this topic,
concluding than even under high stirring and with a small dis-
perse-phase droplet size, the liquid–liquid mass-transfer resist-
ance usually controls the overall kinetics.[18] A poor contact hin-
ders 5-HMF transfer despite it being thermodynamically fa-
vored. Thus, increasing the interfacial area between organic
and aqueous phase is required to promote the intrinsic trans-
port rate.[19]
One of the possibilities to enhance this mass transfer is the
use of amphiphilic materials, such as carbonaceous materials
(nanofibers, nanotubes, graphite, active carbons, etc.). These
kinds of materials tend to adsorb non-hydrophilic molecules in Results and Discussion
the aqueous phase and desorb them in the organic phase. The
Performance in the absence of phase-transfer promoters
equilibrium between these processes depends on the solubility
of the solute in both solvents and on the surface properties of
the adsorbent. Furthermore, the amphiphilic character of these
materials also promotes liquid–liquid dispersion, decreasing
the disperse-phase droplet size and promoting the mass trans-
fer.[20] Although these effects have been explored for organic
Trying to optimize 5-HMF production without promoting the
formation of side products, this reaction was studied with low
acid concentration and a biphasic system (MIBK/water), with
the aim of selectively extracting 5-HMF, the only compound
with larger solubility in the organic phase than in the aqueous
&
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
2
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
phase. Although MIBK has a relative solubility in water
(19 gLꢀ1 at reaction conditions), the MIBK/water ratio (1:1 w/w)
guarantees a perfectly defined biphasic system. In addition to
allowing us to obtain a selective extraction of the desired com-
pound, this methodology introduces an extra advantage for 5-
HMF purification in isolating it as a solid. Despite the scarce
studies about this topic, two different alternatives are pro-
posed in the literature, distillation and crystallization.[24] The
first one was discarded because of the high boiling point of
MIBK (1178C), whereas crystallization is entirely feasible be-
cause of the large difference in freezing points of MIBK
(ꢀ848C) and HMF (308C). To analyze if the presence of the bi-
phasic system has a negative effect on the normal activity, re-
sults plotted in Figures 2 and 3 are compared with the corre-
sponding ones obtained in a monophasic (aqueous) medium.
In this case, reaction was performed by using 0.175 L water as
solvent.
system and a more stable trend in the monophasic system.
Two different reaction steps can produce this compound. On
the one hand, this dimer can be a hydrolytic intermediate be-
tween cellulose and glucose. On the other hand, this com-
pound is also obtained in reactions feeding glucose, produced
by intermolecular dehydration reactions.[25,26] This second route
is proposed as the main one in this case because it is largely
promoted at temperatures close to the glucose melting point
(419 K),[27] and there is no reason to consider a partial hydroly-
sis of cellulose producing cellobiose but not any other oligo-
mer with three or four sugar units, compounds that would be
detected in the liquid phase if they were produced in the reac-
tion. This hypothesis is corroborated by the thermogravimetric
decomposition analyses detailed below. This etherification is a
reversible reaction, congruent with the profiles obtained, and
explains the high amount of dimer observed during all reac-
tions although glucose is consumed in the subsequent steps.
Glucose is also relevant, following in both cases the typical
behavior of an intermediate, with a maximum concentration of
943 ppm in the monophasic system and 550 ppm in the bipha-
sic system (selectivities of 19.5 and 15.7%, respectively), after
which a decreasing trend is clearly observed, a typical conse-
quence of the advance to subsequent steps. Both components,
glucose and cellobiose, are obtained only in the aqueous
phase of the biphasic system. The combined analyses of both
evolutions suggest that reaction in the biphasic system follows
the same mechanism as in the aqueous one, with slower kinet-
ics. This effect is more relevant in the first hydrolytic step (all
these steps require water as solvent, but the first one is the
most sensitive because it requires more severe conditions). As
consequence of the slower kinetics affecting also the sugar de-
hydration, there is more glucose available to produce cello-
biose, justifying the continuous increasing trend obtained in
the biphasic system, whereas the concentration reaches a
stable value with just water as solvent. 5-HMF has an increas-
ing profile in both systems, reaching a maximum of 736 ppm
(selectivity of 15.6%) if the reaction is performed in the aque-
ous phase. However, if the process is performed in a biphasic
configuration, 5-HMF is observed in both phases, with a con-
centration distribution only relevant in the last stages of the
reaction, with 296 and 143 ppm in the aqueous and organic
phase after 24 h, respectively. These results indicate that the
distribution coefficient (expressed as the ratio of concentra-
tions of 5-HMF in both phases) has a value of 0.48. At this
point, it must be remarked that there is no solubility limit in
the concentration range studied because it was experimentally
tested in the laboratory. These data correspond to a 5-HMF
yield of 10.26% in the biphasic system and 11.6% in the
monophasic system.
Figure 2 shows the temporal evolution of the main com-
pounds involved in the reaction. All results shown are the aver-
age value after performing each experiment twice. Experimen-
tal errors were estimated in terms of standard deviation and
were always lower than 5%. It must be highlighted that, ac-
cording to the mechanism and in good agreement with experi-
mental data and previous results presented in the litera-
ture,[14,15] levulinic and formic acid are obtained in equimolar
fashion as the two compounds produced by rehydration of 5-
HMF. Levulinic acid is more relevant as bio-platform molecule
than formic acid, and its production under these conditions
can also be relevant because the reaction conditions are soft
enough to prevent side reactions involving this molecule.
These facts justify that only levulinic acid is plotted in Figure 2
because the concentrations of both reaction products are simi-
lar and follow the same trend.
The main product in both the monophasic (empty symbols)
and the biphasic system (filled symbols) is the glucose dimer
cellobiose, with a continuous increasing trend in the biphasic
As to secondary compounds, such as anhydroglucose (AHG)
as well as levulinic and formic acid, the desired effect is
reached because these compounds are not detected if reaction
is performed in a biphasic system, whereas the concentrations
of these products reach values of 318 and 804 ppm for AHG
and levulinic acid, respectively, after 24 h in the monophasic
system. In fact, the selectivity to levulinic acid rises up to
14.8%, the same value as the desired compound (15% 5-HMF
Figure 2. Temporal evolution of main products involved in cellulose hydroly-
sis (5.83 g) at 413 K with 200 ppm HCl as catalyst. Empty symbols corre-
spond to the aqueous solvent, whereas filled symbols correspond to the bi-
&
*
^
phasic MIBK/water system. Legend: cellobiose ( ); glucose ( ); 5-HMF ( );
~
^
AHG (ꢄ); levulinic acid ( ); and MIBK/water system: in aqueous ( ) and in
^
organic phase ( ) .
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
3
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
selectivity at 24 h). Because both maxima correspond to the
same time, the presence of this side product is the most rele-
vant drawback of the monophasic system, justifying the use of
a biphasic system because the extraction of 5-HMF in MIBK is
100% selective. The low distribution coefficient, however, re-
duces is productivity to only 55% of the one obtained in the
monophasic system. All these results suggest a relevant mass-
transfer limitation, justifying the need for promoting mass
transfer between both phases.
that the biphasic system could be a good option to perform
the reaction considering that acidity and thermal conditions
(and not the organic phase) are the most probable limiting pa-
rameters to obtain a higher conversion.
To determine whether the liquid-phase carbon yield can be
directly related to the cellulose conversion, thermogravimetric
analysis (TGA) was performed, comparing the profiles obtained
with the decomposition profile of the raw material and a
sample of humins. This sample was obtained after a specific
glucose hydrolysis test at very severe conditions (24 h, 413 K,
and 400 ppm of HCl) once the solid phase was recovered by
filtration and dried. Results are plotted in Figure 4. As can be
An accurate analysis of cellulose conversion in these systems
is not possible because of the different phases involved. Be-
cause cellulose is a solid (whereas all the analyzed compounds
are liquids), its conversion must be measured by the difference
in weight before and after the reaction (only liquid samples
are extracted for the temporal evolution). However, the mass
measurement would be conditioned by any solid intermediate
(heavy oligomers) or humins produced, resulting by this tech-
nique in a glucose conversion lower than the real one. Consid-
ering this situation, the evolution of cellulose was analyzed ac-
cording to the “liquid-phase carbon yield” concept defined in
the Experimental Section. Because it is measured considering
the liquid phases (water and MIBK), evolution analysis is possi-
ble, whereas the conversion calculated by mass difference can
also be analyzed at the final point of each reaction.
The temporal evolution of this parameter is depicted in
Figure 3. Results suggest a higher glucose conversion in the
monophasic system (empty symbols) than in the biphasic one
(filled symbols), with the difference most noticeable in the ini-
tial 8 h. However, theoretical conversions after 24 h are very
close: 18.9 and 16.4%, respectively, for both cases. The low dif-
ference observed with the monophasic system between 18
and 24 h suggests that reaction conditions are not harsh
enough to continue hydrolyzing the most stable cellulose
structure once the terminal units have reacted. This result is
congruent with the high crystallinity of this raw material
(94.09% according to XRD). In good agreement with this hy-
pothesis, the biphasic system has the same trend, suggesting
the same final point, but with a clearly slower kinetics. These
results, congruent with the product profiles obtained, indicate
Figure 4. TGA results obtained by analyzing the solid recovered after reac-
tions performed in monophasic (green) and biphasic (red) configuration. Re-
sults compared with the thermal decomposition of pure cellulose (black)
and humins (yellow). Data in continuous lines correspond to derivative ther-
mogravimetry (DTG), whereas dotted lines correspond to the mass loss.
observed, both profiles after the reaction suggest the presence
of cellulose (decomposition temperature at 590–610 K), where-
as the weakest humin signal is at 660 K. The lower mass loss
obtained with the humins sample compared with the other
analyses also suggests the presence of more stable humins de-
posed on the surface, compounds that are not decomposed
even at the maximum temperature of this analysis. However,
final masses of samples after reactions are the same as the
ones obtained with the fresh cellulose, disproving the pres-
ence of any of these side compounds.
The presence of some glucose oligomers in the reaction
samples cannot be totally discarded because these peaks are a
bit wider than the peak of pure cellulose with the maximum
slightly displaced to slower temperatures, a clear signal of par-
tial degradation of the crystalline structure. However, the dif-
ference is not significant enough (10 K in the case of the
monophasic phase) to be considered relevant. According to
the literature, the thermal decomposition of glucose occurs at
temperatures a bit higher than its melting point (419 K), with
some signals detected up to 513 K owing to the loss of water
produced through different oligomerization processes occur-
ring during this decomposition, including cellobiose forma-
tion.[27] According to these results, saccharide decompositions
take place from 420 to 610 K, with the temperature directly re-
Figure 3. Temporal comparison of liquid-phase carbon yield obtained for the
^
reactions performed in aqueous phase ( ) and with the biphasic MIBK/water
^
system ( ).
&
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
4
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
lated to the complexity of their structure (from monosacchar-
ide to polymer).
Taking this into account, the displacement of the main peak
of reaction samples to lower temperatures is suggested to be
related to a small amount of oligomers, with the peaks ob-
tained in the reactions considered as the envelope curve of
cellulose and these small contributions. This slight difference
suggests that, if oligomers are present, those molecules are
closer to the polymer than to the monomer structure. This
result is congruent with the absence of oligomers detected in
the liquid phase, except for cellobiose. According to these
analyses, the absence of humins is corroborated in both cases,
and the clear profiles suggest a good correspondence between
the liquid-phase carbon yield concept and cellulose conver-
sion.
Figure 5. Temporal evolution of liquid-phase carbon yield for reactions per-
&
formed in a biphasic system without any mass-transfer agent ( ), with CNTs
*
^
( ), and with AC ( ).
Carbon materials as phase-transfer promoters
After testing the two-phase configuration, two main conclu-
sions are obtained related to slower kinetics and a poor liquid–
liquid mass transfer. Thus, lower conversions are obtained and
5-HMF is distributed to almost 50% in each solvent, with a low
concentration in MIBK (despite being the only product trans-
ferred to the organic phase). Both problems can be solved if
the phase transfer is improved because some steps of the re-
action are equilibrium steps (glucose isomerization into fruc-
tose and glucose intermolecular dehydration) and the transfer
of HMF can shift these equilibria to the products, enhancing
the overall cellulose conversion.
To improve this transfer, two different carbonaceous materi-
als were tested: CNTs and AC. These two materials are among
the best-known carbonaceous structures and were chosen be-
cause their different properties in terms of surface area to
compare the role of a mesoporous (CNTs, 277 m2 gꢀ1) and a mi-
croporous material (AC, 1005 m2 gꢀ1). The use of other inorgan-
ic materials such as alumina or zeolite has been discarded be-
cause of their inferior adsorption performance as well as their
surface reactivity. All characterization is detailed in a previous
work.[28] In both cases, materials were directly used without in-
troducing any modification in their surface to ensure the ab-
sence of any extra catalytic process that could play a role in
the results. However, the presence of some functional groups
on their surface cannot be completely prevented. According to
the literature, CNTs presents some weak acid groups, whereas
AC also exhibits some oxygen surface groups.[28] In good
agreement, both materials present very similar isoelectric
points (4.19 and 4.63 for CNTs and AC, respectively). It can be
expected that these functionalities have a negligible catalytic
role in this reaction because it requires strong acid sites. To
verify this premise, a preliminary reaction in the absence of
HCl but with the carbon materials was performed, resulting in
negligible cellulose conversion.
&
*
Figure 6. (a) Temporal evolution of cellobiose ( ) and glucose ( ) obtained
in the liquid phase of the biphasic system using no mass-transfer agent
(grey), CNTs (green), and AC (orange). (b) Temporal distribution of 5-HMF (
between phases. Data correspond to aqueous (grey) and organic phase
(black) in absence of any mass-transfer agent; aqueous (orange) and organic
phase (yellow) using AC; and organic phase using CNTs (green), with no
signal for 5-HMF in aqueous phase using CNTs.
^
)
For an initial test, both materials were introduced in 2 wt%
concentration referring to the initial amount of cellulose. The
temporal evolutions of these results are shown in Figures 5
and 6, for the analysis of liquid-phase carbon yield and product
concentration, respectively. According to the analysis of the
two experiments performed for each condition, the relative
errors are always lower than 6%, as shown in Figures 5 and 6.
The expected improvement in the overall reaction is clearly
observed when using CNTs, resulting in higher cellulose con-
version (analyzed in terms of liquid-phase carbon yield) than in
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
5
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
the absence of any mass-transfer agent, with a relative increase
higher than 20% (from 16.5 to 19.95% without and with CNTs,
respectively). A relevant catalytic activity of these CNTs can be
discarded because of the low concentration of functional sites
of these materials, much lower than for the corresponding AC.
Thus, this improvement can be caused by the enhancement in
the transfer of 5-HMF from the aqueous to the organic phase,
also promoting the glucose consumption by the equilibrium
shift. According to this possibility, the amount of glucose
dimer also decreases, and the influence of the amount of CNTs
is low (once the minimum required to promote the complete
transfer is reached, an extra amount has no bearing on the re-
action). To identify the key parameters, reactions with different
amounts of CNTs were performed.
Figure 7. Temporal evolution of 5-HMF concentration obtained as function
~
&
*
^
of the CNT concentration: 0.5 ( ), 1 ( ), 1.5 ( ), and 2% ( ).
In the case of AC, the performance is clearly poorer, with a
final cellulose conversion of 12.18%. These results suggest a
negative role of oxygen sites on the AC surface or a strong ad-
sorption of glucose and cellobiose on the AC surface, as previ-
ously demonstrated in the literature.[29]
Kinetics of the phase transfer in presence of carbon materi-
als: Effect of the pH value and the presence of glucose
Cellobiose and glucose are selectively obtained in the aque-
ous phase with both carbonaceous materials as well as with-
out any mass-transfer agent (Figure 5). The concentration ob-
tained with CNTs is similar to the one without any carbona-
ceous materials, whereas the amount of cellobiose with AC is
significantly lower at all times. This result is congruent with the
suspected strong adsorption of these sugars onto the AC sur-
face. The production of humins, AHG, and levulinic and formic
acid is prevented. These results suggest that 5-HMF is entirely
transferred to the organic phase. For the reaction with CNTs,
no signals of any 5-HMF are detected in the aqueous phase at
any reaction time (distribution coefficient close to infinity). In
contrast, results obtained with AC are even worse than without
adding any mass-transfer agent, affording almost the same
concentration in both cases. This result corresponds to a distri-
bution coefficient close to 1.
To study the extraction of 5-HMF, the time evolution of 5-HMF
concentration in aqueous and MIBK phase was analyzed for
the four CNT concentrations (0.5, 1, 1.5, and 2%). The evolution
of 5-HMF concentration in the organic phase is compared in
Figure 8. The evolution obtained upon adding AC was also in-
cluded. In all cases, the transfer reaches an equilibrium after
300 min. The maximum concentration obtained is higher in
presence of the carbonaceous materials, with slight differences
as function of the concentration and the material used. Thus,
the partition coefficient (organic/aqueous concentration under
stationary conditions) evolves from 1.09 (absence of any mass-
transfer promoter) to 1.23 (1.5% CNTs). However, the main dif-
ferences are related to the speed at which these equilibriums
are reached because the partition coefficients reached with
0.5, 1, and 2% of CNTs and AC are very similar (from 1.15 to
1.17).
The infinite distribution coefficient suggests that 2% CNTs
could be too much, in such a way that lower CNT concentra-
tions could be sufficient to maximize the extraction. Although
the adsorption is more relevant in the case of AC, this cannot
be discarded in the case of CNTs because the total final
amount of 5-HMF obtained (371 ppm) is lower than the total
amount obtained without CNTs in the reaction (439 ppm) or
for using a monophasic system (736 mg). Thus, further experi-
ments were performed by decreasing the CNT concentration
(0.5, 1, and 1.5%). The main difference expected concerns the
5-HMF concentration; this evolution is plotted in Figure 7.
Despite the similar profiles obtained, best results are obtained
with 1.5% CNT (386 ppm). This result corresponds to a 43.3%
selective conversion of cellulose into HMF (corresponding to a
9% total yield of 5-HMF from cellulose), 23% more than with-
out the mass-transfer agent and a value higher than typically
obtained under comparable conditions (low temperature and
low catalytic loading in a biphasic medium).[22] Lower values of
5-HMF concentration were obtained with both lower and
higher concentrations, requiring an in-depth study of 5-HMF
extraction to identify if it is only explained in mass-transfer
terms or if the overall reaction kinetics control the process.
The in-depth analysis of these results requires analyzing the
kinetics of the process. Considering that all steps involved in
this process only take place in the aqueous medium, the ap-
pearance of 5-HMF in the organic phase, as well as its faster or
Figure 8. Temporal evolution of 5-HMF concentration in the organic phase
in the extraction studies. Experimental points correspond to no mass-trans-
*
*
*
*
*
fer agent ( ); 0.5% CNTs ( ); 1% CNTs ( ); 1.5% CNTs ( ); 2% CNTs ( ); 2%
*
AC ( ).
&
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
6
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
slower temporal evolution must be controlled by the transfer
between aqueous and organic phase. According to this prem-
ise, the modelling of this mass-transfer process was done con-
sidering overall mass-transport coefficients (Kw and Ko for the
water and organic phase, respectively). Thus, the 5-HMF trans-
port rate [molLꢀ1 minꢀ1] can be calculated as a function of the
effective transport coefficients (Ki ꢄa, with a being the effective
coefficient and i indicating the phase [minꢀ1]) according to the
following equations for water (w) and MIBK (o) phase [Eqs. (1)
and (2)]:
dCwHMF
dt
dCoHMF
dt
ꢀ
ꢁ
ꢀ
ꢁ
w
w
w
*
ð1Þ
ð2Þ
¼ ꢀ KHMF ꢁ a ꢁ CHMF ꢀ CHMF
Figure 9. Comparison of volumetric mass-transfer coefficients [minꢀ1] as
function of the percentage of carbonaceous materials in the interface.
ꢀ
ꢁ
ꢀ
ꢁ
¼ KoHMF ꢁ a ꢁ C
ꢀ CoHMF
o;
*
HMF
*
^
Orange symbols ( ) correspond to CNTs; green symbols ( ) to AC.
in which C refers to concentration, and terms with asterisks
correspond to the concentration in each phase under equilibri-
um conditions. In the experimental fit, these values correspond
to stable concentrations obtained after 24 h of extraction, as a
particular value for each experiment (5-HMF concentrations in
both phases do not undergo any evolution after 300 min in
any case). Considering the principle of mass conservation,
these expressions are equal. Thus, both volumetric mass-trans-
port coefficients can be related by the partition coefficient,
HHMF, defined as the ratio between 5-HMF in the organic and
aqueous phase once the equilibrium is reached [Eq. (3)]:
volumetric transfer coefficients (real conditions in the reaction
studied). In the second case, this modification could be
checked by studying the 5-HMF transfer when sugars are also
present, evaluating if their adsorption on the AC surface hin-
ders the mass transfer. These two conditions were tested with
both CNTs and AC, comparing the final volumetric mass-trans-
fer coefficients in Figure 10.
A clear improvement is observed when working under
acidic conditions (striped bars) with both CNTs and AC. Both
coefficients reach values twice as high as those under neutral
pH. This improvement is a clear consequence of the positive
effect of acidic conditions in liquid–liquid extractions.[30] This
result suggests that the protonation of the carbon functional
groups increases their phase-transfer promotion effect, which
is more marked for AC because of its larger concentration of
these sites.
KoHMF ¼ HHMF ꢁ KwHMF
ð3Þ
This fact allows us to study the whole mass-transfer process
by just analyzing the evolution of one phase. In this case, be-
cause 5-HMF is selectively extracted to the organic solvent,
this phase was chosen to evaluate this phenomenon. Taking
into account that the initial concentration in the organic phase
is zero, the integration of the concentration evolution in the
organic phase is simplified in Equation (4):
The presence of other compounds in the aqueous phase
can modify the mass-transfer process because these molecules
can be adsorbed, decreasing the free surface available to the
n
o
Ko ꢁa ꢁt
Þ ꢂ
1 ꢀ eꢀ
ð4Þ
o
o
½ð
*
HMF
CHMF;t ¼ CHMF
ꢁ
Experimental data were fitted according to this model, ob-
taining good correlation coefficients (R2 >0.92 in all cases). The
values of volumetric mass-transfer coefficients in the organic
phase as function of the CNT percentage are plotted in
Figure 9. In all cases, the carbon balances considering both
phases are close to 100%, discarding any relevant permanent
adsorption phenomenon even in the case of AC. Values ob-
tained are in good agreement with the concentration of 5-
HMF in the organic phases of reaction media, suggesting that
extraction is promoted with 1.5% CNTs. However, the high co-
efficient observed with AC (very similar to results of 1 and
1.5% CNTs) contrasts with its poor behavior in the reactions.
This apparently incongruent behavior of AC can be caused
by two main reasons: a strong pH dependence or a negative
influence of sugar adsorption. In the first case, results obtained
with 200 ppm HCl in the reaction medium would modify the
Figure 10. 5-HMF overall mass-transfer coefficient values [minꢀ1] obtained
with CNTs (green) and AC (orange) as function of the extraction conditions:
neutral pH selective extraction of 5-HMF (striped bars); acidic pH selective
&
&
&
extraction of 5-HMF ( and ); and acidic pH with co-presence of glucose (
&
and ).
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
7
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

Full Papers
Hydrochloric acid (37%) was purchased from Fisher Chemical. The
cellulose was characterized by different techniques to determine
the main properties that could affect this reaction. Thus, inductive-
ly-coupled plasma (ICP) analysis was performed to identify the
atomic bulk composition by using an octapole HP-7500c. The cellu-
lose was dissolved in HNO3 (1%) and Rh was used as internal stan-
dard. The crystallographic structure was determined by XRD on a
Philiphs X’Pert Pro diffractometer, working the with the CuKa line,
in the range 2q=5–308. The crystallinity index (CI) was calculated
by using the Segal equation [Eq. (5)]:
5-HMF transfer. To investigate this effect, the co-presence of
glucose in the extraction medium was analyzed, chosen be-
cause it is the major component in the reaction medium. Re-
sults indicate that the co-presence of glucose (darkest bars)
has a negative effect on both coefficients, suggesting a com-
petitive process between mass transfer and glucose adsorp-
tion. This decrease is much less evident in the case of CNTs
(relative decrease of 16.7%), whereas KoHMF ꢄa decreases more
than 50% for AC, obtaining a final value very similar to the
one at neutral pH. This situation is owing to the high affinity
between sugars and the AC surface, as previously mentioned
in the literature.[2] Because these conditions are similar to the
ones in the real reaction medium, this adsorption is proposed
as the main cause of the inferior behavior of AC as selective
mass-transfer promoter in this reaction, making this negative
effect more relevant than the improvement caused by the
acidic conditions. In contrast, CNTs are a good mass-transfer
promoter, making the transport of 5-HMF to the organic phase
fast enough to promote the overall process, preventing the
long-time contact of 5-HMF with acid sites that can produce
undesired secondary reactions (humins and acids), and obtain-
ing a 5-HMF productivity 2.7 times higher than the one ob-
tained in the absence of mass-transfer agents. In addition, all
5-HMF is extracted to the organic phase, whereas less than
50% is efficiently extracted with the water/MIBK system.
Ic ꢀ Ia
ð5Þ
CI ¼
Ic
in which Ic is the intensity of the maximum crystalline peak and Ia
is the minimum intensity between two crystalline peaks. Two dif-
ferent carbonaceous materials were used as mass-transfer agents:
CNTs provided by Dropsens and AC (GC-900, supplied by Chemi-
Vall). These materials were characterized in depth in a previous
study.[28]
Reactions and sample analyses
Reactions were performed in a 0.5 L stirred autoclave reactor (Au-
toclave Engineers EZE seal) equipped with a proportional-integral-
derivative (PID) temperature controller and a back-pressure regula-
tor. The reactor was loaded with a biphasic mixture of MIBK and
water (350 mL; 175 mL water and 218 mL MIBK, 1:1 w/w) with cel-
lulose (5.83 g, 50–80 mm) and HCl (200 ppm) as catalyst. The reac-
tor was pressurized to 10 bar with N2 and heated to the reaction
temperature, 413 K. If the reaction was performed in the presence
of carbonaceous materials, different mass loadings of CNTs or AC
were added to the system (values detailed in the corresponding
section of the main text).
Samples were taken from the sampling port by using a 0.45 mm
Nylon syringe filter. The aqueous phase was analyzed by HPLC
(1200 Series, Agilent) with a refraction index detector (G1362A RI).
The method was optimized by using a Hi-Plex H Column (Agilent)
as stationary phase and 5 mm H2SO4 solution as the mobile phase.
The organic phase was analyzed by capillary GC in a Shimadzu GC-
2010 equipped with a flame ionization detector. A 15 m long CP-Sil
5 CB column was used as the stationary phase. For both instru-
ments, quantitative responses were determined by using standard
calibration mixtures. Each sample was analyzed twice by HPLC or
GC, as required, obtaining a good reproducibility of these analyses,
with relative errors lower than 4% in all cases.
Conclusions
The use of carbon nanotubes (CNTs) as mass-transfer promot-
ers clearly improves the transformation of cellulose into 5-hy-
droxymethylfurfural (5-HMF) in a water/methyl isobutyl ketone
(MIBK) biphasic system. Considering that the acidic conditions
required to promote cellulose hydrolysis are also responsible
for 5-HMF degradation into humins and levulinic acid, the ex-
traction of this compound to an organic phase is needed. A se-
lective extraction is proposed by using a water/MIBK system,
but the 5-HMF formation rate of this process is decreased by
the slower liquid–liquid mass-transfer kinetics. CNTs are pro-
posed as promising mass-transfer promoters, enhancing the
extraction kinetics more than 3.7 times, mainly under acidic
conditions. Because several equilibrium steps are involved in
the process, this extraction also results in an increased produc-
tivity (270 times higher). The use of activated carbon (AC) in-
stead of CNTs is discarded owing to the competition between
5-HMF and glucose for the adsorption sites (the first one is re-
versible, but the second one causes partial blockage of the
AC). With this approach, 5-HMF can be selectively produced
from cellulose without need for a complex catalyst, toxic sol-
vents, or severe conditions.
The cellulose conversion is calculated in terms of liquid-phase
carbon yield, a concept involving the theoretical cellulose required
to obtain all compounds detected in the liquid phase. This concept
only involves the real conversion of cellulose to products of the
main route (cellobiose, glucose, 5-HMF, AHG, formic acid, and levu-
linic acid). This value, analyzed in carbon terms, is calculated ac-
cording to Equation (6):
P
P
½V ꢁ ðni ꢁ CiÞꢂaqueous þ ½V ꢁ ðni ꢁ CiÞꢂorg
hc ¼
ð6Þ
mcellulose
0:4421 ꢁ M
cellulose
Experimental Section
in which hc is the liquid-phase carbon yield, V is the volume of
each solvent, Ci the molar concentration of each compound de-
tected in each liquid phase (water or organic phase), ni the
number of carbons in the molecule of compound i, mcellulose the
mass of cellulose introduced in the reactor at the initial point,
Mcellulose the molar mass of cellulose (162 gmolꢀ1), and 0.4421 corre-
Materials
Microcrystalline cellulose (CAS: 9004-34-6), as well as d-(+)-glucose
(ꢃ99.5%), d-(ꢀ)-fructose (ꢃ99%), 5-HMF (ꢃ99%), formic acid
(98%), and levulinic acid (98%) were provided by Sigma–Aldrich.
&
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
8
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Papers
[5] J. G. Hu, K. Gourlay, V. Arantes, J. S. Van Dyk, A. Pribowo, J. N. Saddler,
ChemSusChem 2015, 8, 901–907.
[6] a) P. Laopaiboon, A. Thani, V. Leelavatcharamas, L. Laopaiboon, Biore-
sour. Technol. 2010, 101, 1036–1043; b) A. M. Adel, Z. H. A. El-Wahab,
A. A. Ibrahim, M. T. Al-Shemy, Bioresour. Technol. 2010, 101, 4446–4455;
c) H. Hꢅkansson, P. Ahlgren, Cellulose 2005, 12, 177–183.
[7] D. Garcꢀs, E. Dꢁaz, S. OrdꢂÇez, Ind. Eng. Chem. Res. 2017, 56, 5221–
5230.
[8] a) Y. Sun, J. Cheng, Bioresour. Technol. 2002, 83, 1–11; b) P. Kumar, D. M.
Barrett, M. J. Delwiche, P. Stroeve, Ind. Eng. Chem. Res. 2009, 48, 3713–
3729; c) S. Van De Vyver, J. Geboers, P. A. Jacobs, B. F. Sels, ChemCat-
Chem 2011, 3, 82–94.
[9] I. Sꢆdaba, M. L. Granados, A. Riisager, E. Taarning, Green Chem. 2015, 17,
4133–4145.
[10] a) M. A. Kougioumtzis, A. Marianou, K. Atsonios, C. Michailof, N. Nikolo-
poulos, N. Koukouzas, K. Triantafyllidis, A. Lappas, E. Kakaras, Waste Bio-
mass Valorization 2018, 9, 2433–2455; b) A. Mukherjee, M.-J. Dumont,
R. Raghavan, Biomass Bioenergy 2015, 72, 143–183.
sponds to the atomic percentage of carbon in this cellulose, ac-
cording to results obtained by ICP analysis of the raw material
(44.21% C, 6.2% H, 49.59% O).
The yield of 5-HMF is calculated based on the mass of this com-
pound divided by the mass of cellulose converted, as shown in
Equation (7):
CHMF;water ꢁ Vwater þ CHMF;MIBK ꢁ VMIBK
ð7Þ
fHMF
¼
hc ꢁ V
where CHMF,water and CHMF,MIBK are the concentration of HMF in the
water and MIBK phases; Vwater, VMIBK, and V are the volumes of aque-
ous phase, MIBK, and the total one, respectively. Selectivities are
calculated as the carbon included in each compound divided by
the total amount of carbon present in all compounds quantified in
the liquid phase.
Solids obtained after each reaction were dried and analyzed with a
thermogravimetry–differential scanning calorimetry (TG–DSC) in-
strument (Setaram, Sensys) by using a-alumina as inert reference
material. The samples (20 mg) were treated in a nitrogen flow
(20 mLminꢀ1) with a temperature program of 5 Kminꢀ1 from 298
to 873 K.
[11] D. Garcꢀs, L. Faba, E. Dꢁaz, S. OrdꢂÇez, ChemSusChem 2019, 12, 924–
934.
[12] Z. Xue, M.-G. Ma, Z. Li, T. Mu, RSC Adv. 2016, 6, 98874–98892.
[13] H. Lin, Q. Xiong, Y. Zhao, J. Chen, S. Wang, AIChE J. 2017, 63, 257–265.
[14] G. Gliozzi, A. Innorta, A. Mancini, R. Bortolo, C. Perego, M. Ricci, F.
Cavani, Appl. Catal. B 2014, 145, 24–33.
[15] J. Shen, C. E. Wyman, AIChE J. 2012, 58, 236–246.
[16] a) T. F. Wang, M. W. Nolte, B. H. Shanks, Green Chem. 2014, 16, 548–572;
b) C. Li, Z. Zhang, Z. K. Zhao, Tetrahedron Lett. 2009, 50, 5403–5405;
c) S. Dutta, S. De, B. Saha, Biomass Bioenergy 2013, 55, 355–369; d) V. da
Silva Lacerda, J. B. Lꢂpez-Sotelo, A. Correa-Guimaraes, S. Hernꢆndez-
Navarro, M. Sꢆnchez-Bascones, L. M. Navas-Gracia, P. Martꢁn-Ramos, E.
Pꢀrez-Lebena, J. Martꢁn-Gil, Bioresour. Technol. 2015, 180, 88–96.
[17] a) Y. J. Pagꢆn-Torres, T. F. Wang, J. M. R. Gallo, B. H. Shanks, J. A. Dumesic,
ACS Catal. 2012, 2, 930–934; b) B. Op de Beeck, M. Dusselier, J. Gebo-
ers, J. Holsbeek, E. Morrꢀ, S. Oswald, L. Giebeler, B. F. Sels, Energy Envi-
ron. Sci. 2015, 8, 230–240; c) R. Weingarten, A. Rodrꢁgeuz-Beuerman, F.
Cao, J. S. Luterbacher, D. M. Alonso, J. A. Dumesic, G. W. Huber, Chem-
CatChem 2014, 6, 2229–2234.
[18] a) K. M. Roberts, D. M. Lavenson, E. J. Tozzi, M. J. McCarthy, T. Jeoh, Cellu-
lose 2011, 18, 759–773; b) D. B. Hodge, M. N. Karim, D. J. Schell, J. D.
McMillan, Bioresour. Technol. 2008, 99, 8940–8948.
[19] M. T. Jimarꢀ, F. CazaÇa, A. Ramirez, C. Royo, E. Romeo, J. Faria, D. E. Re-
sasco, A. Monzꢂn, Catal. Today 2013, 210, 89–97.
Extraction analyses
The mass-transfer kinetics as well as adsorption/desorption studies
were performed by reproducing the reaction conditions in terms
of solvent ratio and HCl concentration. In each experiment, 5-HMF
(0.18 g) was dissolved in water (25 mL). Once the corresponding
amount of mass-transfer agent was added (same concentrations as
those studied in the reaction experiments), MIBK (31.25 mL) was
added. The temporal evolution of 5-HMF was analyzed by GC (or-
ganic phase) and HPLC (aqueous phase). To identify a possible
competitive adsorption of 5-HMF and glucose, similar analyses
were performed with the same total mass of organics (0.18 g) and
a 1:1 (w/w) distribution of both compounds.
[20] a) E. LorenÅon, D. C. Ferreira, R. R. Resende, K. Krambrock, Appl. Catal. A
2015, 505, 566–574; b) D. Shi, J. Faria, T. N. Pham, D. E. Resasco, ACS
Catal. 2014, 4, 1944–1952; c) X. Yang, X. Wang, J. Qiu, Appl. Catal. A
2010, 382, 131–137; d) A. A. S. Oliveira, I. F. Teixeira, L. P. Ribeiro, E. Lor-
enzon, J. D. Ardisson, L. Fernꢆndez-Outon, W. A. A. Mancedo, F. C. C.
Moura, Appl. Catal. A 2013, 456, 126–134.
[21] a) F. Delbecq, Y. Wang, C. Len, Mol. Catal. 2017, 434, 80–85; b) N.
Sweygers, J. Harrer, R. Dewil, L. Appels, J. Cleaner Prod. 2018, 187,
1014–1024; c) F. Delbecq, C. Len, Molecules 2018, 23, 1973–1988.
[22] H. M. Mirzaei, B. Karimi, Green Chem. 2016, 18, 2282–2286.
[23] Y.-R. Zhang, N. Li, M.-F. Li, Y.-M. Fan, RSC Adv. 2016, 6, 21347–21351.
[24] T. Stꢅhlberg, J. S. Jensen, A. Riisager, J. M. Woodley (Danmarks Tekniske
Universitet, Lyngby, Denmark), WO2013024162A1, 2012.
Acknowledgements
This work has been founded by Research Projects of the Regional
Government of Asturias (Project Reference FC-GRUPIN-IDI/2018/
000116) and Spanish Ministry of Economy and Competitiveness
(CTQ2017-89443-C3-2-R).
Conflict of interest
The authors declare no conflict of interest.
[25] A. T. Pedersen, R. Ringborg, T. Grotkjær, S. Pedersen, J. M. Woodley,
Chem. Eng. J. 2015, 273, 455–464.
[26] H. M. Pilath, M. R. Nimlos, A. Mittal, M. E. Himmel, D. K. Johnson, Food
Chem. 2010, 58, 6131–6140.
[27] F. ꢇrsi, J. Therm. Anal. 1973, 5, 329–335.
Keywords: carbon nanotubes · cellulose · green chemistry ·
hydrolysis · methyl isobutyl ketone
[28] Y. PatiÇo, E. Dꢁaz, S. OrdꢂÇez, Chemosphere 2015, 119, S124–S130.
[29] I. Abe, K. Hayashi, M. Kitagawa, Carbon 1983, 21, 189–191.
[30] K. Osseo-Asare, Adv. Colloid Interface Sci. 1991, 37, 123–173.
[1] a) S. Sadula, A. Athaley, W. Q. Zheng, M. Ierapetritou, B. Saha, ChemSu-
sChem 2017, 10, 2566–2572; b) J. Remꢂn, J. Randall, V. L. Budarin, J. H.
Clark, Green Chem. 2019, 21, 284–299; c) L. Faba, E. Dꢁaz, S. OrdꢂÇez,
Renewable Sustainable Energy Rev. 2015, 51, 273–287.
[2] D. Klemm, B. Heubleim, H.-P. Fink, A. Bohn, Angew. Chem. Int. Ed. 2005,
44, 3358–3393; Angew. Chem. 2005, 117, 3422–3458.
[3] X. Tong, Y. Ma, Y. Li, Appl. Catal. A 2010, 385, 1–13.
Manuscript received: May 9, 2019
[4] a) I. Alam, S. De, T. S. Khan, M. A. Haider, B. Saha, Ind. Crops Prod. 2018,
123, 629–637; b) J. Cueto, L. Faba, E. Dꢁaz, S. OrdꢂÇez, Appl. Catal. B
2017, 201, 221–231.
Revised manuscript received: June 4, 2019
Version of record online: && &&, 0000
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
9
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
L. Faba, D. Garcꢀs, E. Dꢁaz, S. OrdꢂÇez*
Carbon saves 5-HMF: It is well known
that biphasic systems are efficient for
avoiding side reactions of 5-hydroxyme-
thylfurfural (5-HMF) in cellulose acid hy-
drolysis. However, liquid–liquid mass
transfer largely decreases the reaction
rate. The addiction of carbon materials
(for example carbon nanotubes, CNTs)
effectively promotes the interphase
mass transfer of 5-HMF, increasing the
overall reaction rate.
&& – &&
Carbon Materials as Phase-Transfer
Promoters for Obtaining 5-
Hydroxymethylfurfural from Cellulose
in a Biphasic System
&
ChemSusChem 2019, 12, 1 – 10
www.chemsuschem.org
10
ꢃ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!