Catalytic conversion of xylose to furfural by p-toluenesulfonic acid (Ptsa) and chlorides: Process optimization and kinetic modeling

Article abstract of DOI:10.3390/molecules26082208

Furfural is one of the most promising precursor chemicals with an extended range of downstream derivatives. In this work, conversion of xylose to produce furfural was performed by employing p-toluenesulfonic acid (pTSA) as a catalyst in DMSO medium at moderate temperature and atmospheric pressure. The production process was optimized based on kinetic modeling of xylose conversion to furfural alongwith simultaneous formation of humin from xylose and furfural. The synergetic effects of organic acids and Lewis acids were investigated. Results showed that the catalyst pTSA-CrCl₃·6H₂ O was a promising combined catalyst due to the high furfural yield (53.10%) at a moderate temperature of 120^? C. Observed kinetic modeling illustrated that the condensation of furfural in the DMSO solvent medium actually could be neglected. The established model was found to be satisfactory and could be well applied for process simulation and optimization with adequate accuracy. The estimated values of activation energies for xylose dehydration, condensation of xylose, and furfural to humin were 81.80, 66.50, and 93.02 kJ/mol, respectively.

Full text of DOI:10.3390/molecules26082208

molecules
Article
Catalytic Conversion of Xylose to Furfural by p-Toluenesulfonic
Acid (pTSA) and Chlorides: Process Optimization and
Kinetic Modeling
Muhammad Sajid 1,2,3,
*
, Muhammad Rizwan Dilshad 4 , Muhammad Saif Ur Rehman 5, Dehua Liu 2
and Xuebing Zhao 2,
*
1
Faculty of Materials and Chemical Engineering, Yibin University, Yibin 644000, China
Institute of Applied Chemistry, Department of Chemical Engineering, Tsinghua University,
2
Beijing 100084, China; dhliu@tsinghua.edu.cn
Department of Chemical Engineering, University of Gujrat, Gujrat 50700, Pakistan
Institute of Chemical Engineering and Technology, University of the Punjab, Lahore 54590, Pakistan;
3
4
rizwandilshad@gmail.com
5
Department of Chemical Engineering, Khwaja Fareed University of Engineering and Information Technology,
Abu Dhabi Road, Rahim Yar Khan 54000, Pakistan; saifrehman1224@yahoo.com
*
Correspondence: engr.sajid80@gmail.com (M.S.); zhaoxb@mail.tsinghua.edu.cn (X.Z.);
Tel.: +86-178-1026-4474 (M.S.)
Abstract: Furfural is one of the most promising precursor chemicals with an extended range of
downstream derivatives. In this work, conversion of xylose to produce furfural was performed by
employing p-toluenesulfonic acid (pTSA) as a catalyst in DMSO medium at moderate temperature
and atmospheric pressure. The production process was optimized based on kinetic modeling of
xylose conversion to furfural alongwith simultaneous formation of humin from xylose and furfural.
The synergetic effects of organic acids and Lewis acids were investigated. Results showed that the
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Sajid, M.; Rizwan Dilshad,
M.; Saif Ur Rehman, M.; Liu, D.; Zhao,
X. Catalytic Conversion of Xylose to
Furfural by p-Toluenesulfonic Acid
(pTSA) and Chlorides: Process
catalyst pTSA-CrCl3
·
6H2O was a promising combined catalyst due to the high furfural yield (53.10%)
◦
at a moderate temperature of 120 C. Observed kinetic modeling illustrated that the condensation of
furfural in the DMSO solvent medium actually could be neglected. The established model was found
to be satisfactory and could be well applied for process simulation and optimization with adequate
accuracy. The estimated values of activation energies for xylose dehydration, condensation of xylose,
and furfural to humin were 81.80, 66.50, and 93.02 kJ/mol, respectively.
Optimization and Kinetic Modeling.
Molecules 2021, 26, 2208. https://
doi.org/10.3390/molecules26082208
Academic Editors: Cesar Mateo and
Farid Chemat
Keywords: catalysis; chlorides; furfural; kinetics; lewis acid; p-toluenesulfonic acid; xylose
Received: 8 March 2021
Accepted: 6 April 2021
Published: 12 April 2021
1. Introduction
Furfural is a high-value furan aldehyde, with a wide range of industrial uses, especially
in the production of pharmaceuticals, polymers, resins, solvents, fine chemicals, fuel
additives, and biofuels [1–4]. Xylose is an encouraging feedstock for furfural synthesis
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
because of the process simplicity and product selectivity [5,6].
Numerous reports have described the different aspects of xylose dehydration to
furfural [
steered the evaluation of comprehensive reaction kinetics in order to illuminate the detailed
reaction mechanism [ ]. It has been observed that catalytic conversion of xylose resulted in
the simultaneous production of furfural and some miscellaneous products (Scheme 1).
Mineral acids [ ], modified solid acids [10], metal oxides [11], and some organic
acids [12], have been used for the production of furfural from xylose by employing water,
organics, ionic liquids, and their combinations as solvents. Generally, mineral acids are
applied in organic synthesis because of the higher process yields, and heterogeneous solid
1,3,5]. Ample interest in the development of techno-economical processes has
7
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
8,9
acids are used because of their high recycling efficiencies [
9,13]. Besides these advantages,
mineral acids are less selective owing to associated environmental concerns [14]. On the
Molecules 2021, 26, 2208. https://doi.org/10.3390/molecules26082208
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2208
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other hand, solid acid catalysts typically lack recyclability after the first cycle due to fouling
and preferred adsorption of stable process intermediates; hence these are less efficient for
mass production, especially for biomass-derived components [15,16]. Ionic liquids were
found to be relatively efficient reaction media having integrated functions of catalysts
as well as solvents [17]. Nevertheless, high cost, low recovery, and poor recycling are
substantial hindrances for their industrial applications [18]. Interest in the replacement
of mineral acids by organic acids as the catalyst is increasing to overcome the stubborn
problems of environmental pollution [19]. Hence, organic acids have been acknowledged as
prospective catalysts for the dehydration of sugars. Additionally, only a 50% furfural yield
has been achieved in the industrial mineral acid-catalyzed furfural production process [20].
Hazardous waste problems due to HCl and/or H2SO4, and high-energy requirements
◦
(the reaction temperature is typically 200 C) are some observed drawbacks. Hence,
an alternative method with mild reaction conditions, improved energy efficiency, and
decreased environmental problems are needed. Biocatalysis is also an alternative non-toxic
route to make the desired product under milder reaction conditions; however, slow reaction
rates and delicate process control are vital for an excellent yield [21–23]. Meanwhile,
organic acids have low corrosion impact and are reasonably selective considering the
process economics and rate of reaction [24]. Moreover, thermal decomposition of organic
acids yield relatively less toxic and recyclable components like HCOOH, CO2, and fumaric
acid avoiding waste stream contamination [24]. Thus, organic acids can be considered
as potential chemical catalysts having the benefits of homogenous catalysis as well as
relatively low environmental toxicity.
Scheme 1. Acid-catalyzed conversion of xylose to furfural.
Recently, the kinetics of xylose dehydration have been discussed applying heteroge-
neous catalysts [25,26] and homogenous catalysts, including mineral acids [27] and organic
acids [12]. Although biphasic solvent systems are now widely used, kinetic investigations
of xylose dehydration were mostly studied in the aqueous phase at high temperature and
high-pressure [28].
Voluminous attention have been paid to mineral acids to be used in homogenous
catalysis for xylose conversion to furfural. HCl was used for the biphasic kinetic analysis
of xylose dehydration, applying microwave heating [29]. Results indicated a faster rate of
xylose decomposition to humin than xylose dehydration to furfural. Shifting the reaction
medium from monophasic to biphasic by the addition of an organic solvent improved the
furfural yield from 30% to 85%. However, it was noted that the reaction kinetics remained
unchanged in both reaction media either microwave irradiation or conventional heating
were applied [29]. H3PO4 and H2SO4 were also used as homogeneous mineral acid cata-
lysts [30,31]. Similar benefits are also available with organic acids, but these have not been
thoroughly investigated. The use of organic acids may be the most attractive reaction sys-
tem considering the furfural production along with integrated organosolv fractionation [12].
Kinetic dehydration of xylose employing formic acid was performed at high temperatures
◦
(130–200 C). Similar to formic acid, acetic acid [32] and p-toluenesulfonic acid [25] are also

Molecules 2021, 26, 2208
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equally efficient for dehydration reactions. However, a comprehensive systematic kinetic
investigation of the process and process mechanism has not been performed.
It has been witnessed that the addition of chlorides can improve the process efficiency
of sugars dehydration to furan aldehydes [27
homogenous catalysis similar to mineral acids with low environmental toxicity [
,
33], and organic acids have the benefits of
34]. pTSA,
2,
being solid at room temperature, has the advantage of easy recovery using established
precipitation techniques [25]. Therefore, in this work, a combination of organic acids and
chlorides was investigated using organic solvents for the eco-friendly production of furfural.
The dependency of the product yield of different process parameters was optimized
using kinetic modeling. The role of Lewis acid addition was calculated considering the
maximum furfural yield. Additionally, the reaction system was optimized considering
the kinetic parameters and maximum process yield. A comparative evaluation of the
experimental results and corresponding kinetic data elucidated that CrCl3
best promoter for the pTSA-catalyzed conversion of xylose to produce furfural. The
pTSA-CrCl3 6H2O catalyst is cheap and ecologically friendly as compared to mineral
·
6H2O is the
·
and solid acid catalysts. The proposed process thus has the potential to be operated as
a tool for advanced optimization to achieve the maximize furfural yield from biomass-
derived sugars.
2. Results and Discussion
2.1. Aqueous Phase Dehydration of Xylose into Furfural
Synthesis of furfural from xylose was investigated using optimized reaction condi-
tions from our previous experiments (conversion of C6 sugars to HMF) and then again
optimized [33,34]. Water is a preferred solvent because of process economics, ecological
concerns, high sugar dissolving capacity, and versatile physio-chemical properties. Ap-
plying organic acids in pure form, a meager furfural yield was observed, as the applied
temperature was 100 ◦C due to pressure constraints (Table S1). Köchermann et al. studied
the aqueous phase xylose conversion using sulfuric acid as a homogeneous catalyst [35].
Applying an aqueous phase high-pressure reactor, the obtained furfural yield was 36.3% in
◦
15 min with 91.8% xylose conversion at 200 C. A 52.0% of the furfural yield was achieved
by increasing the temperature to 220 ◦C, which showed the positive impact of higher tem-
peratures [35]. High temperatures can be secured either by using a high-pressure reaction
system or by using high boiling point solvents. Therefore, a higher pressure (6.0 MPa) was
maintained to achieve a high temperature in the water medium [35]. Continuous provision
of such a high pressure has economic and safety concerns, so we have fixed the pressure
as atmospheric pressure considering the industrial safety and energy expenditure, so an
increase in pressure is not a sustainable preference and hence was not elaborated.
2.2. Effect of Lewis Acid Addition
It has been observed that the addition of Lewis acid improved the isomerization and
dehydration efficiency of the organic acid-catalyzed process when used in glucose dehydra-
tion reactions [33]. Similar results were also reported in the mineral acid-catalyzed xylose
dehydration process as the aqueous phase HCl catalyzed furfural◦yield was increased to
38.7% from 28.8% with the addition of 13.5 mM CrCl3 6H2O at 145 C [36]. Thus, different
·
Lewis acids were added to investigate the synergy of organic acid and Lewis acid combi-
nations. CrCl3 6H2O gives the highest furfural yield in an aqueous medium (Figure 1A),
similar to published literature [14 36]. Concentration effects were investigated by applying
0.1 M to 0.6 M of CrCl3 6H2O and keeping all the other parameters constant. An increase
·
,
·
in catalyst concentration was unswervingly reciprocated in the xylose conversion and
furfural yield.

Molecules 2021, 26, 2208
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Xylose conversion
Furfural yield
Furfural selectivity
Xylose conversion
Furfural yield
Furfural selectivity
60
50
40
30
20
10
0
100
80
60
40
20
0
A
B
0.1
0.3 0.4 0.5 0.6 1.0
A
0B.2
C
D
E
F
G
NH4Cl CrCl3.6H2O AlCl3
CrCl3 conc. (M)
Figure 1. Xylose conversion into furfural. Reaction conditions: 0.5 M xylose and 1.0 M pTSA in 50 mL H2O with reaction
◦
temperature 100 C and constant stirring at 200 RPM. (
A
) 0.1 M of different Lewis acids, (
B
) different concentrations
of CrCl3·6H2O.
The maximum furfural yield of 30.05% was recorded with 82.10% xylose conversion
at 0.5 M concentration of CrCl3 6H2O after this decreasing trend was observed (Figure 1B).
·
These results directed the positive impact of Lewis acid addition on furfural yield. It
has been proved that metal halides strongly affect the dehydration rate in an aqueous
medium [37]. Xylose dehydration increased with a decrease in the water activity and water
activity is decreased by the addition of halides in the following order Cl− >Br− > I−
[37].
When the effect of trivalent chlorides was considered, Cr gives the maximum efficiency
following the order of CrCl3 > AlCl3 > InCl3 > FeCl3 [38]. Molecular dynamics simula-
tions elucidated that CrCl3
pH
4 [39]. Cr(H2O)5OH2+ is the more reactive species and affects the dehydration of sug-
ars positively. Therefore the synergetic effects of HCl-CrCl3 6H2O and H3PO4-CrCl3 6H2O
gave the best results for the conversion of glucose to HMF [39 40]. Similar results were
also obtained in our earlier findings employing a combined pTSA-CrCl3 6H2O catalyst [33].
In the same way, these results also illuminated that CrCl3 6H2O is a more active support-
·
6H2O produces Cr(H2O)5OH2+ when combined with water at
≤
·
·
,
·
·
ing catalyst than AlCl3 and NH4Cl. It is further speculated that side reactions of xylose,
reaction intermediates, and produced furfural form unwanted side products that were
responsible for the lower furfural yield in water medium [37,38]. To further check the
effects of temperature and solvent medium on the product selectivity, water was replaced
with organic solvents.
Kinetic analysis, as shown in Table S2 and Figure S1, also confirms the catalytic supe-
riority of the pTSA-CrCl3
increased with the addition of Lewis acids in the order of CrCl3
no Lewis acid. An increase in k2obs and k3obs was also perceived along with k1obs with the
addition of CrCl3 6H2O, which is also a confirmation of the active catalytic performance
·
6H2O co-catalyzed process. The principal rate of reaction (k1obs
)
·
6H2O > AlCl3 > NH4Cl >
·
of the selected reaction system. It has been observed that trivalent chromium salts are
highly efficient in aqueous as well as in aprotic solvent-based dehydration reactions [41].
Therefore, a gradual increase in k1obs, with an increase in CrCl3 6H2O concentration was
·
experienced (Table S3).
The increase in the prime rate of reaction is similarly reciprocated by an increase in
product yield. The change in k2obs and k3obs was not uniform. It can be witnessed from
Table S3, that k1obs increased from 1.16
×
10−3 min−1 to 1.70
×
10−3 min−1 by changing
the CrCl3 6H2O concentration from 0.4 M to 0.5 M and a similar increase was also noted in
·

Molecules 2021, 26, 2208
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k
2obs, whereas, contrarily k3obs decreased from 1.14
×
10−4 min−1 to 5.21
×
10−11 min−1
.
Kinetic results showed the narrow variation of k1obs and k2obs, while a relatively high
variation in k3obs (10−12–10−4) was recorded. At low concentration, k2obs were higher
than k1obs; whereas, at high concentration, the observed rate constants were nearly equal
(
k1obs ≈
k2obs). This observation indicated the higher tendency of xylose decomposition to
side products (denoted as humin) than furfural formation in water. Weingarten et al., also
reported that the rate of aqueous phase xylose decomposition to side products (k2obs) was
higher in the HCl-catalyzed process [29]. Results reported by Lamminpä ä et al., employing
HCOOH as a catalyst also suggested a similar phenomenon [12]. The observed rates were
nearly equal and calculated k1obs and k2obs were 6.85
×
10−4 min−1 and 2.28 × 10−4 min−1
,
respectively [12]. The calculated kinetic parameter in this study (Tables S2 and S3) are
in good consistency with the reported literature values. The kinetic plots (Figure 1 and
Figure S2) show a good agreement between the model’s predicted values and experimen-
tally determined values. All the goodness of fit (R2) values is >0.95, which proves the
accuracy of the developed model.
2.3. Conversion of Xylose to Furfural in Organic Medium
Optimized parameters were applied to check the quantitative effects of polar aprotic
organic solvents (DMSO and DMF) on furfural yield. As indicated in Figure 2A, high
product yield was observed in the DMSO solvent medium as compared to DMF and water
solvent media. Quantitatively, the observed experimental yield order was DMSO > H2O
> DMF. Surprisingly, the furfural yield in the aqueous phase (30.05%) was a little higher
than in the DMF phase (29.74%). However, the reaction time needed in the aqueous phase
(6 h) was longer than in the DMF phase (1 h); whereas, the maximum furfural yield was
obtained in only 0.5 h when DMSO was used as a solvent medium. It has been noted that
the use of water as a monophasic solvent medium for the conversion of xylose to furfural
accelerates the side reactions which adversely affects the product selectivity [36
effects were also observed in our previous experiment of the conversion of hexoses to
HMF [33 34]; this phenomenon further proved the preferred solvation property of DMSO
,42]. Similar
,
toward furan aldehydes. Therefore, the furfural yield in theaqueous phase was lower
as compared to DMSO solvent medium. It was also observed that a higher substrate
concentration exceeded the product yield in homogeneous catalysis [43]. In this study, high
xylose concentration may be a reason for this increase in yield. DMSO, an organic solvent,
could solve this issue. DMSO was also the best organic solvent for the HMF production
process from C6 sugars because of its steadiness even at high-temperature and superior
solvation toward furan aldehydes [33,34]. It may be partly because DMSO can effectively
introverted the furfural decomposition into unwanted side products [43].
Kinetic analysis of xylose conversion in organic medium has been performed similarly
to the aqueous phase process. The kinetic curves of xylose, furfural, and humin concentra-
tions are provided in Figure 3, and observed rate constants are listed in Table 1. The kinetic
comparison proved the solvation superiority of DMSO. The highest k1obs was observed
when DMSO was used as a solvent medium and k1obs increases in the order of DMSO >
DMF > H2O. However, when the rate constants of organic solvents were compared, high
k
1obs, lower k2obs, and k3obs were found with DMSO than DMF (Table 1). The magnitude
of k1obs and k2obs is larger than k3obs; therefore, furfural decomposition could be neglected
under these process conditions. These results indicated that the rate constants in water
media are far less than in the DMF medium, but the recorded product yield was a little
higher in the water medium. This is because of the high proportional effect of the prime
rate of reaction to undesired rate of reaction in the water medium than in the DMF medium.
The data, as shown in Table 1 shiwed that a higher reaction selectivity (SF = k1obs/k2obs
)
could be obtained in the DMSO medium than in DMF and water media. Therefore, DMSO
was found to be the most promising solvent for this organic acid-catalyzed dehydration of
xylose to furfural. Dehydration reactions are ominously influenced by the reaction tem-

Molecules 2021, 26, 2208
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perature, and it is generally accepted that furfural moieties cannot undergo degradation
reactions in the vapor phase [42].
Xylose conversion
Xylose conversion
Furfural yield
Furfural yield
120
A
B
Furfural selectivity
100
Furfural selectivity
100
80
60
40
20
0
80
60
40
20
0
DMSO
Water
DMF
110 120 130 140 150 160
Temp (oC)
Xylose conversion
Furfural yield
Furfural selectivity
120
100
80
60
40
20
0
C
0.1 0.2 0.3 0.4 0.5
CrCl3 conc (M)
Figure 2. Xylose dehydration◦into furfural. Reaction conditions: (
A
) 0.5 M xylose, 1.0 M pTSA, and 0.5 M CrCl3
·
6H2O in
◦
50 mL solvents heated at 120 C (for water 100 C), (
B
) 0.5 M xylose, 1.0 M pTSA and 0.5 M CrCl3
·
6H2O in 50 mL DMSO
6H2O in 50 mL DMSO
heated at different temperatures, (
C
) 0.5 M xylose, 1.0 M pTSA and different concentrations of CrCl3
·
◦
and 120 C.
X-Cal
A
B
C
FF-Cal
HUM-Cal
X-Exp
FF-Exp
HUM-Exp
0.6
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
0.4
0.2
0.0
0
100 200 300 400 500
Time (min)
0
100 200 300 400 500
Time (min)
0
100 200 300 400 500
Time (min)
Figure 3. Kinetic plots of pTSA-catalyzed dehydration of xylose into furfural. Reaction conditions: 0.5 M xylose, 1.0 M
pTSA, and 0.5 M CrCl3 6H2O catalysts in 50 mL solvents, (
) DMF at 120 ◦C; ( ) DMSO at 120 ◦C; ( ) water at 100 ◦C.
Lines represent calculated data whereas symbols represent experimental values. X: xylose; FF: furfural; HUM; humin.
·
A
B
C

Molecules 2021, 26, 2208
7 of 17
Table 1. Fitted reaction parameters for xylose conversion into furfural in different solvent mediums. Reaction conditions:
◦ ◦
0.5 M Xylose, 1.0 M pTSA, and 0.5 M CrCl3 6H2O catalysts in 50 mL solvent at 120 C (water 100 C) and stirring at 200 rpm
·
for 8 h.
Goodness of Fit (R2)
SF
Max Yield
−1
Rate Constant (min
)
Solvent
k1obs
k2obs
k3obs
Xylose
Furfural
Humin
k1obs/k2obs
Ymax
−3
−4
−11
DMF
DMSO
Water
0.029
0.033
0.053
0.047
0.997
0.999
0.974
0.961
0.973
0.986
0.989
0.990
0.922
0.547
0.701
0.572
29.74 ± 1.1
40.35 ± 1.6
30.1 ± 0.9
2.04 × 10
3.20 × 10
−3
−3
1.70
×
10
2.97
×
10
5.21 × 10
The established reaction parameters (0.50 M xylose, 1.0 M pTSA, and 0.50 M CrCl3 6H2O)
·
were then used to appraise the dependency of furfural yield and selectivity on temperature
employing DMSO as a reaction medium. The process temperature was synchronized from
110 ◦C to 160 ◦C, and the results are presented in Figure 2B. Kinetic data are presented
in Table 2 and the corresponding kinetic plots in Figure S3. The results proved that the
established kinetic model can successfully explain the process kinetics of this reaction
system. The developed kinetic plots revealed the significant impact of temperature on
furfural selectivity. It can be observed from the results that furfural yield increased with an
◦
◦
increase in temperature from 110 C to 120 C. However, a further increase in temperature
from 120 ◦C to 160 ◦C affects inversely, reducing the furfural yield. This shoddier furfural
yield may be attributed to the higher rate of xylose decomposition to humin than xylose
dehydration to furfural [42]. The rate of furfural decomposition (k3obs) remains substantially
low throughout the experiment. This can also be wittnessed by the concentration profiles
showing a continuous increase in the furfural concentration followed by a minor decline
next to a peak value. However, a high rate of furfural decomposition (k3obs) is observed at
high reaction temperatures, especially beyound 140 ◦C. All the rate constants improved
with an increase in temperature, and k2obs were higher than k1obs at all temperatures except
◦
130 C. The objective reaction (conversion of xylose to furfural) had a smaller rate constant
(k1obs) than humin formation (k2obs) but the difference was only 5%, so it can be assumed
that xylose has an equal rate of dehydration and decomposition at all temperatures in the
◦
DMSO medium. The humin formation was lowest at 120 C, which was increased with the
temperature rise. Therefore, the◦magnitude of the rate of xylose decomposition to humin
(k2obs) was 0.022 min−1 at 120 C, and it increased gradually and the highest value of
◦
0.2948 min−1 was observed at 160 C. Correspondingly, a change in color from light yellow
to dark brown is observed which proved the increased byproduct formation in the reaction
mixture. A kinetic study of xylose dehydration in high-pressure liquid water also reported
a rise in k2obs with increased temperatures [44]. A similar increase in rate constants with
temperature escalation has also been reported in the literature [12,32,41]. The magnitude
of k1obs and k2obs changed from 0.00322 min−1 and 0.00128 min−1 to 0.0356 min−1 and
0.0297 min−1, respectively, with a temperature rise from 180 ◦C to 220 ◦C [44]. Hence, it
can be validated that temperature strongly stimulates the product selectivity in the pTSA-
CrCl3 6H2O catalyzed process of xylose dehydration to furfural in the DMSO medium, and
·
a careful balance is required between the objective reaction and side reactions to achieve a
high product selectivity.

Molecules 2021, 26, 2208
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Table 2. Fitted reaction parameters for xylose conversion into furfural in DMSO. Reaction conditions: 0.5 M Xylose, 1.0 M
pTSA, and 0.5 M CrCl3 6H2O catalyst in 50 mL DMSO and stirring at 200 rpm with variable reaction temperatures for 5 h.
·
Goodness of Fit (R2)
Max Yield
tYmax
−1
Rate Constant (min
)
◦
Temp ( C)
k1obs
k2obs
k3obs
Xylose
Furfural
Humin
Ymax
min
−4
−4
−4
−3
−3
−3
110
120
130
140
150
160
0.0208
0.033
0.0854
0.1286
0.1740
0.2792
0.0220
0.047
0.0549
0.1368
0.1866
0.2948
0.9990
0.9999
0.9999
0.9999
0.9999
0.9999
0.979
0.973
0.997
0.919
0.995
0.931
0.988
0.990
0.999
0.973
0.999
0.983
33.5 ± 1.1
40.4 ± 1.6
34.1 ± 1.1
33.8 ± 1.3
34.4 ± 1.7
34.8 ± 1.2
240
30
30
15
15
15
1.70
3.70
9.45
2.57
4.03
2.72
×
×
×
×
×
×
10
10
10
10
10
10
Metal chlorides were found equally efficient in water as well as in organic solvent
media. However, a high concentration of Lewis acids may result in an additional financial
burden. Therefore, the impact of Lewis acid contents on furfural selectivity was further
optimized in the DMSO medium, as the purpose was to check the process efficiency under
reduced concentration conditions. Hence, the CrCl3 6H2O level was varied between 0.1 M
·
to 0.5 M, and obtained results are presented in Figure 2C, and kinetic parameters are
shown in Table 3 and Figure S4. Surprisingly, the results suggested an inverse relation
between furfural yield and Lewis acid concentration, and the highest furfural yield of
52.60% was attained with 0.1 M CrCl3
observed in the kinetic parameters (Table 3). The principal rate constant (k1obs) increases
inversely with CrCl3 6H2O concentration. The magnitude of k1obs and k2obs increased
to 0.0508 min−1 and 0.0542 min−1 from 0.033 min−1 and 0.047 min−1, respectively, by
decreasing the CrCl3 6H2O concentration from 0.5 M to 0.1 M. This increase in product
selectivity with a decrease in system acidity proves the negative impact of high acidity. By
decreasing CrCl3 6H2O from 0.5 M to 0.1 M, a proportional growth of 54.0% and 15.1% in
1obs and k2obs was noted, respectively. This significant difference in a proportional increase
in rate constants (k1obs than k2obs) was straightly reciprocated in furfural yield.
·
6H2O concentration. Similar phenomena could be
·
·
·
k
Table 3. Fitted reaction constant for xylose conversion into furfural in DMSO. Reaction conditions: 0.5 M xylose, 1.0 M
pTSA, and 0.1–0.5 M CrCl3
6H2O catalyst in 50 mL DMSO and stirring at 200 rpm, with reaction temperature of 120 ◦C
·
for 5 h.
Goodness of Fit (R2)
Xylose Furfural Humin
Max Yield
−1
Rate Constant (min
)
CrCl3·6H2O (M)
tYmax (min)
SF
k1obs
k2obs
k3obs
Ymax
−4
−4
−4
−4
−4
0.1
0.2
0.3
0.4
0.5
0.0508
0.0509
0.0360
0.0349
0.0330
0.0542
0.0605
0.0508
0.0519
0.047
0.9990
0.9999
0.9999
0.9999
0.9999
0.9543
0.9737
0.9778
0.9814
0.973
0.9538
0.9797
0.9905
0.9929
0.990
52.6 ± 1.3
48.4 ± 1.4
45.1 ± 1.3
43.1 ± 2.2
40.4 ± 1.6
60
60
90
90
30
0.94
0.84
0.71
0.67
0.70
5.49 × 10
6.76 × 10
3.01 × 10
2.55 × 10
3.70 × 10
The reaction selectivity (SF = k1obs/k2obs) also increased with a decrease in CrCl3 6H2O
·
concentration and the highest value of 0.94 is observed at the lowest concentration (Table 3).
The magnitude of k3obs remains in the range of 10−4, so it can be concluded that a change
in Lewis acid concentration does not affect the furfural decomposition reaction appreciably
and furfural decomposition remains very low at all proportion. Kinetic plots, as shown
in Figure S4, verified this hypothesis, which is shown by constant furfural concentration
after reaching a peak value. Thus, it can be established that the principal reaction of xylose
dehydration into furfural in the DMSO solvent medium increased with a decrease in
CrCl3·6H2O concentration in contrast to the aqueous phase reaction.

Molecules 2021, 26, 2208
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2.4. Comparison of Different Organic Acids
The impact of different organic-Lewis acid catalytic combinations in the DMSO
medium was further explored. The kinetic profiles of xylose, furfural, and byproducts
(humin) are presented in Figure 4, and the resultant parameters are enumerated in Table 4
.
The high R2 (
≥
0.95) again proved the accuracy of the developed kinetic model. The ob-
served rate constants elucidated that the xylose conversion is in the sequence of pTSA
> oxalic acid > maleic acid > succinic acid > malonic acid (Table 4). However, the maxi-
mum product obtained shows little deviation from this trend. The values of k1obs were in
the range of 10−3–10−2 min−1. Corresponding reaction selectivity for xylose conversion
(k1obs/k2obs) was determined to be 0.94, 0.71, 0.52, 0.0.66 and 0.51, respectively. It has been
observed the pTSA-CrCl3 6H2O catalyst obtained the maximum furfural yield in DMSO
·
medium, similar to the aqueous phase process. Furfural yields obtained in the order of
pTSA > oxalic acid> succinic acid > maleic acid > malonic acid.
X-Cal
FF-Cal
HUM-Cal
X-Exp
FF-Exp
0.6
0.6
A
C
B
0.4
0.2
0.0
0.4
HUM-Exp
0.2
0.0
0
100
200
300
300
300
0
100
200
300
Time (min)
Time (min)
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
D
0 E
100
200
Time (min)
0
100
200
300
Time (min)
0.6
0.4
0.2
0.0
0
100
Time (min)
200
Figure 4. Kinetic plots of dehydration of xylose in◦to furfural. Reaction conditions: 0.5 M xylose, 1.0 M organic acid, and
0.1 M CrCl3 6H2O catalysts in 50 mL DMSO at 120 C for 5 h; (A) pTSA; (B) Oxalic acid; (C) Succinic acid; (D) Malonic acid;
·
(
E
) Maleic acid. Lines represent calculated data whereas symbols represent experimental values. X: xylose; FF: furfural;
HUM; humin.
Table 4. Fitted reaction parameters for xylose conversion into furfural in DMSO. Reaction conditions: 0.5 M xylose, 1.0 M organic
◦
acid, and 0.1 M CrCl3·6H2O catalysts in 50 mL DMSO and stirring at 200 rpm, with reaction temperature of 120 C for 5 h.
Goodness of Fit (R2)
Max Yield
−1
Rate Constant (min
)
Organic Acid
tYmax (h)
SF
k1obs
k2obs
k3obs
Xylose Furfural Humin
Ymax
−4
pTSA
0.0508
0.0129
0.0056
0.0062
0.0042
0.0542
0.0183
0.0084
0.0120
0.0081
0.9990
0.967
0.986
0.973
0.885
0.9543
0.983
0.9718
0.9
0.9538
0.8826
0.8816
0.7568
0.4205
52.60 ± 1.3
42.64 ± 1.2
41.95 ± 1.5
37.1 ± 1.2
32.41 ± 2.2
1
5
5
5
5
0.94
0.71
0.66
0.52
0.51
5.49 × 10
−12
−14
−14
−14
Oxalic acid
Succinic acid
Maleic acid
Malonic acid
1.30 × 10
9.30 × 10
2.20 × 10
2.20 × 10
0.9486

Molecules 2021, 26, 2208
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The reaction mechanism can be explained assuming a 1st order conversion of reactant
X to product F, as X
given by the following expression:
→
F for xylose conversion having an apparent differential rate equation
d[F]
dt
= kR[X]
(1)
Here kR is the observed rate constant. As the reaction is catalytic and proceeded with
the addition of homogenous catalysts; therefore, incorporating catalytic effect [H+] with
quasi-stationary distribution, the kinetic model could be expressed as [34]:
ꢀ
ꢁ
kR1
d[F]
dt
= k
[X] H+
(2)
R2 (kR2 + kR−1
)
From Equations (1) and (2), it is evident that kR depends upon [H+] linearly with the
following relationship:
ꢀ
ꢁ
kR1kR2
R2 + kR−1
kR =
H+
(3)
k
pTSA is a relatively strong organic acid with a pKa1 value of
−
2.8; hence complete
dissociation can be assumed, whereas, for other organic acids, the aqueous phase [H+] can
be estimated by the following equation [45]:
p
ꢀ
ꢁ
H+
≈
CAcidKa1
(4)
here CAcid represents the acid catalyst concentration. Considering this relationship
(
Equation (4)), the [H+] of the used organic acid catalysts was calculated and plotted
in Figure 5. The results demonstrated a good linear relationship (R2 = 0.9786) and prove
the accuracy of the developed model.
0.06
0.05
k1=0.0312[H+]
R2 = 0.9786
0.04
pTSA
0.03
0.02
Succinic Acid
0.01
0.00
0.0
0.2
0.4
0.6
0.8
1.0
[H+] estimated by pKa1
Figure 5. The dependency of experiment-determined k1obs on [H+] of the used organic acid catalysts
in DMSO solvent medium.
The comparison of pKa1 indicated pTSA as the strongest acid (pKa1
=
−
2.8) among all
the organic acids applied which results in its almost complete dissociation providing the
highest [H+] in the reaction system. The rate of reaction is roughly directly proportional to
proton concentration {[H+]} in acid-catalyzed reactions, as proved in Equation (3) (Figure 5).
Furthermore, the synergetic effects of CrCl3
sugars dehydration reactions. Hence, the high xylose conversion was achieved in only one
·
6H2O addition contribute positively to the

Molecules 2021, 26, 2208
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hour whereas other organic acid catalysts required up to five hours to achieve similar results.
These variations indicate that every mixed acid system do not have similar synergistic
effect [33
,
39,40]. Therefore, the high xylose conversion may be attributed to the combined
effects of pTSA-CrCl3
·
6H2O. Oxalic acid (pKa1 = 1.27), likewise exhibited relatively better
catalytic efficacy for xylose dehydration and gave a furfural yield of 42.64%. Interestingly,
succinic acid having the lowest acidity (pKa1 = 4.2) gives a higher product yield than maleic
acid (pKa1 = 1.9) and malonic acid (pKa1 = 2.83). It has been proved that the correlation
of [H+] and k is linear (Equation (10)); this anomaly can be explained by several reasons.
Firstly, the synergy of succinic-CrCl3 6H2O positively affects the principal reaction (xylose
·
dehydration to furfural), which is also confirmed by the high reaction selectivity (SF = 0.66).
Secondly, the synergetic effect may have changed the combined acidity at the reaction
temperature. It has also been reported that numerous other factors were also contributing
and partially influenced the dehydration mechanism [42,46]. Furthermore, the variation in
pKa1 of the organic acids with reaction temperature and solvent medium should also be
considered and a detailed investigation is further required.
The literature indicates that metal chlorides (FeCl3, AlCl3, and CrCl3) mostly give
30–45% furfural yield depending upon the system concentrations, in the absence of
Brønsted acids [38]. Similarly, only a 33.8% furfural yield was achieved by employing
CrCl3 6H2O as a sole catalyst. In this study, the high furfural yield with high sugar
·
contents is credited to the synergetic effects of pTSA-CrCl3 6H2O catalyst in the DMSO
·
solvent medium.
The substrate concentration effects were further examined by varying the initial xylose
concentrations between 0.25–1.0 M while keeping all the other parameters constant (1.0 M
pTSA, 0.10 M CrCl3
6H2O, 120 ◦C, and atmospheric pressure). The obtained results and
·
fitted kinetic parameters are provided in the Supplementary Information (Table S4, and
Figure S5). The time course concentration profiles indicate that there was no significant
influence of initial xylose concentration on the conversion rates, while a minimal impact can
be noted on the product yield. When the xylose concentration was changed from 0.25 M to
0.5 M, the furfural yield decreased from 53.1% to 52.6%. However, with increasing the initial
concentration to 1.0 M, the furfural yield decreased to 42.4%. At a high concentration of
xylose, the negative effect on furfural yield is due to multiple reasons [8,17]. Simultaneous
decomposition of xylose and furfural to humin along with reactive intermediates are
probable reasons. It can be observed from the concentration profiles (Figure S5) that xylose
conversion is a relatively fast reaction, and more than 80% conversion was achieved in the
first 15 min. However, the complete transformation was delivered after 2 h, whereas, the
maximum furfural yield was attained in between 1–1.5 h. After achieving the maximum
value, a slight decrease in furfural concentration can be observed. Furthermore, at high
concentrations, some process intermediates remained unconverted, and a longer reaction
time was required for their complete conversion. Probably, a non-uniform mass distribution
at high concentration results in a decreased number of effective molecular collisions [7,47].
It has been observed that prolonged reaction time results in an increased rate of humin
formation from furfural and other process intermediates [37]. Hence, the cumulative effect
of high substrate concentration resulted in decreased product yield.
The relationship between temperature, the rate constant, and catalyst concentration
could be elaborated using the modified form of the Arrhenius equation:
−E
a
k = Ae( )Caαcid
(5)
(6)
RT
Ea
ln k = ln A −
+ α ln CA
RT
Consequently, the kinetic parameters for k1obs, k2obs, and k3obs can be calculated
employing multiple linear regression and using the data from Tables 2 and 3. Multiple
linear regression when applied gives satisfactory goodness of fit as R2
Figure S4). The objective reaction (xylose dehydration to furfural) has higher activation
≥
0.85 (Table 5 and

Molecules 2021, 26, 2208
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energy than the crucial side reaction (decomposition of xylose to humin). Nevertheless, the
effect of catalyst concentration was more significant on xylose dehydration as compared to
xylose decomposition, as revealed by the high α value for k1obs than k2obs
:
ꢂ
ꢃ
81800
RT
1.35
k1obs = 8.91 × 108 exp −
C
(7)
CrCl3·6H2O
ꢂ
ꢃ
66500
RT
k2obs = 2.60 × 107 exp −
C
C
(8)
(9)
0.204
CrCl3·6H2O
ꢂ
ꢃ
93020
RT
k3obs = 7.13 × 108 exp −
0.268
CrCl3·6H2O
Table 5. Calculated kinetic factors for xylose dehydration to furfural (k1obs), xylose condensation
to humin (k2obs), and furfural decomposition to humin (k3obs) by multiple linear regression for
pTSA-CrCl3·6H2O catalyzed dehydration of xylose in DMSO solvent medium.
R2
−1
A
Ea (kJ/mol)
α
kobs (min
)
8.91 × 108
2.60 × 107
7.13 × 108
k1obs
k2obs
k3obs
81.8
66.5
93.02
1.35517
0.20398
0.26768
0.855
0.962
0.880
The comparison of kinetic results with published literature is appropriate but chal-
lenging because of discrepancies in experimental conditions. Generally, the observed rate
constants are a function of temperature, pressure, and the reaction system. Activation
energy data for xylose dehydration compiled in the Supplementary Information (Table S5)
showed a great variation in values (76–152 kJ mol−1). Lamminpä ä et al., explored the kinet-
ics of formic acid-catalyzed xylose dehydration and the calculated activation energies were
152 kJ mol−1 and 161 kJ mol−1 for k1obs and k2obs, respectively [12]. The reported activation
energies were almost two times higher than this system because the reaction conditions
applied were entirely different. The applied xylose concentration was only 0.08–0.2 M,
whereas the reaction temperature was 130–200 ◦C. Low substrate concentration and high
reaction temperature generally increase the product selectivity in organic synthesis [48].
Similarly, the Ea determined by Chen et al., was 108.6 kJ mol−1 and 105 kJ mol−1 for k1obs
and k2obs, respectively, when acetic acid (0.5 M) was used in a high-pressure reactor [32].
It has been observed that the reaction medium plays a critical role in the reaction energy
barrier [49]. The observed activation energy changes from 145 kJ mol−1 to 114 kJ mol−1
when the reaction medium was changed from the aqueous phase to the GVL phase under
identical reaction conditions (H2SO4 catalyst and 145–175 ◦C). Henceforward, it can be
concluded that system reactivity and selectivity dynamically demonstrate the activation
energies in any reaction. For a systematic comparison, all the parameters must be identical;
otherwise, activation energy comparison seems inappropriate.
3. Materials and Methods
3.1. Materials
Xylose (99.0%), p-toluenesulfonic acid (pTSA, 99.80%), oxalic acid (99.70%), maleic
acid (99.70%), malonic acid (97.50%), and succinic acid (99.8.0%) were bought from Shang-
hai Aladdin Biotechnology Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF,
99.5%), dimethyl sulfoxide (DMSO, 99.9%), isopropyl alcohol (IPA, 99.80%), and Lewis
acids (CrCl3
Chemical Works (Beijing, China). All the analytical standards such as furfural (99.9%), Lyx-
ose (99.9%), xylulose ( 99.0%), and xylose ( 99.7%) were supplied by Shanghai Aladdin
Biotechnology Co., Ltd. Water with very high resistivity ( 18.25 mega-Ohm (M )) was
obtained from the available laboratory purification system.
·
6H2O, AlCl3, and NH4Cl) were obtained from a local supplier namely Beijing
≥
≥
≥
Ω

Molecules 2021, 26, 2208
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3.2. Conversion of Xylose to Furfural
The conversion experiment was performed similarly to the experimental procedure
of conversion of C6 sugars to 5-hydroxymethylfurfural as defined in our previous pub-
lications [33,34]. Briefly, a 3-neck Pyrex glass round bottom flask equipped with a total
reflux condenser was used as a reactor and 50 mL reaction volume was used. The prede-
fined molar concentration of xylose and required amount of catalyst was added into the
reactor and homogenized using a stirrer. Organic acids such as oxalic acid (pKa1 = 1.27),
maleic acid (pKa1 = 1.90), malonic acid (pKa1 = 2.83), succinic acid (pKa1 = 4.20), and pTSA
(
pKa1 = −2.80) were used as catalysts, whereas, CrCl3
·
6H2O, AlCl3, and NH4Cl were used
as Lewis acid co-catalysts. The process was optimized by employing different reaction pa-
rameters in order to achieve the maximum product yield. Sample collection was performed
after a predetermined time interval using a syringe pump and samples were preserved in
the refrigerator until analyzed.
4. Analytics
4.1. Product Quantification
Product concentrations were determined using a HPLC system (Shimadzu, Kyoto,
Japan) equipped with an Aminex® HPX-87H strong acid cation exchange resin column
◦
(
300 mm × 7.8 mm, Bio-Rad, Hercules, CA, USA) at 65 C with a RID-10A differential
refractive index (RI) detector. The analysis was performed with 0.8 mL/min of M/200
sulfuric acid solution. Ultrapure water ( 18.25 M ) was used for the dilution of the
≥
Ω
samples and filtration was performed using 0.22-micron syringe filters. An auto-injection
module was applied and 20 micro litter (µL) sample volume was injected for analysis.
Process efficiency was calculated based on xylose conversion (Xx), furfural yield (YF),
furfural selectivity (SF), and carbon balance (CB). These parameters were calculated using
Equations (10)–(13). CB was calculated in order to estimate the number of undesirable
by-products and summarized as humin (HUM):
ꢄ
ꢅ
Xylose (M)
Xylose in feed (M)
ꢅ
Xylose conversion (XX) = 1 −
× 100
(10)
(11)
ꢄ
Furfural (M)
Furfural yield (YF) =
× 100
Xylose in feed (M)
ꢄ
ꢅ
Furfural yield (YF)
Furfural selectivity (SF) =
× 100
(12)
(13)
Xylose conversion (XX)
Carbon balance (CB) = (5 × XX) − (5 × YF) = YHUM
4.2. Development of Kinetic Models
The conceivable reaction route of xylose conversion is outlined in Scheme 1 and the
detailed reaction mechanism in Scheme S1. Although different reaction routes for xylose
dehydration have been under consideration for a long time; however, the reaction as
described in Scheme 1 is mostly considered the principal route [10,27,29]. It has been
observed that xylose conversion produces furfural and unwanted side products (HUM),
simultaneously. Correspondingly, the decomposition of furfural into side products is also
inevitable. The discrete reaction details are intricate and an exact mechanism is still not
fully developed but the reactions (Equations (14)–(16)) can be used as simplified reaction
pathways following the Scheme 1. Accordingly, the subsequent kinetic models could be
established as:
dCX
= −(k1obs + k2obs)CX
= k1obsCX− k3obsCF
(14)
(15)
(16)
dt
dCF
dt
dCHUM
dt
= k2obsCX + k3obsCF

Molecules 2021, 26, 2208
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where k1obs, and k2obs, are the reaction rate constants of the conversion of xylose into
furfural and xylose decomposition to side products (humin), k3obs is the decomposition
rate of furfural to undesired products (humin). Humin (HUM) is the sum of condensation
products and quantified using a material balance as defined in Equation (13) [50]. CX, CF,
and CHUM denote the molar concentrations of xylose, furfural, and humin, respectively.
4.3. Estimation of Kinetic Parameters
Experimental data was used for the evaluation of the kinetic parameters using MAT-
LAB R2016a. The estimation of kinetic parameters such as rate constants (k) and activation
energy (Ea), was performed using numerical integration of Equations (14)–(16) by ode45
function similar to our previously published procedure [33,34]. The parametric optimiza-
tion was achieved employing the least-squares method. The numerical calculation of the
Arrhenius factor (A) and activation energy (Ea) was done by MATLAB R2016a software
employing the Arrhenius equation.
5. Conclusions
Catalytic transformation of xylose into furfural was elaborated over different organic
acids in aqueous as well as in organic solvent medium. The reaction yield was optimized
by navigating different reaction conditions and adding different chlorides as Lewis acid
◦
co-catalysts. An extensive range of reaction temperatures (110–160 C) and Lewis acid
concentrations (0.1–0.5 M) was applied for process optimization. Furfural yield and
xylose conversion were compared based on experimental results and kinetic modeling. A
kinetic model was developed comprising the reactions of dehydration of xylose to furfural,
decomposition of xylose to humin, condensation of furfural to humin. Results illustrated
the catalytic inferiority of pure pTSA in the aqueous medium; however, much improvement
in furfural selectivity was achieved by adding various chlorides as Lewis acid co-catalysts.
The addition of CrCl3
·
6H2O as a co-catalyst improved the reaction yield up to 30.1% at
100 ◦C in an aqueous medium. Replacement of the reaction medium from water to DMSO
gives the maximum furfural yield of 53.1% using pTSA and CrCl3 6H2O in a molar ratio
·
of 10:1 in 60 min at 120 ◦C. Modeling results illustrated the model accuracy with high R2.
The model predicted values validated that dehydration of xylose to furfural (k1obs) and
decomposition of xylose to humin (k2obs) are the key reactions. k1obs and k2obs were found
to display similar oscillations, while condensation of furfural to humin (k3obs) was in the
−3
−4
range of 10 to 10 with narrow vacillations contingent on the reaction temperature and
chloride concentration. It was observed that k1obs increased with an increase in temperature
and decreased with an increase in Lewis acid concentration. However, k2obs increased with
an increase in temperature and displays a narrow oscillation with an increase in Lewis acid
concentration. The activation energies for xylose dehydration, xylose decomposition, and
condensation of furfural were determined to be 81.8, 66.5, and 93.0 kJ/mol, respectively. It
was also observed that xylose dehydration to furfural was more sensitive toward Lewis
acid concentrating than xylose decomposition and furfural condensation to humin. The
obtained results further suggest that furfural selectivity can be improved by depressing the
rate of xylose condensation.
Supplementary Materials: The following are available online, Scheme S1: Reaction mechanism
of acid-catalyzed dehydration of xylose to furfural. Table S1. Xylose transformation into furfural.
Table S2. Fitted kinetic constants of organic acid-catalyzed conversion of xylose into furfural in the
water medium. Table S3. Fitted kinetic constants of organic acid-catalyzed conversion of xylose into
furfural in the water medium. Table S4. Fitted kinetic constants of organic acid-catalyzed conversion
of xylose into furfural in DMSO solvent medium. Table S5. Comparison of the activation energy for
acid-catalyzed conversion of xylose into furfural in different solvent systems: Figure S1. Kinetic plots
of pTSA-catalyzed conversion of xylose into furfural in the water medium. Figure S2. Kinetic plots
of pTSA-catalyzed conversion of xylose into furfural in water solvent medium. Figure S3. Kinetic
plots of pTSA-catalyzed conversion of xylose into furfural in the DMSO solvent medium. Figure S4.
Kinetic plots of pTSA-catalyzed conversion of xylose into furfural in the DMSO solvent medium.

Molecules 2021, 26, 2208
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Figure S5. Kinetic plots of organic acid-catalyzed conversion of xylose into furfural in the DMSO
solvent medium.
Author Contributions: Conceptualization, methodology, experimentation, M.S. and M.R.D.; formal
analysis, writing—original draft preparation, writing—review, and editing, M.S. and M.S.U.R.; Su-
pervision, X.Z. and D.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by China Scholarship Council (No. 2017SLJ020385), National
Natural Science Foundation of China (No. 21878176), and Yibin University China.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available in the article and
supplementary information.
Acknowledgments: We sincerely acknowledge the support provided by the Key Laboratory of In-
dustrial Biocatalysis, Institute of Applied Chemistry, Department of Chemical Engineering, Tsinghua
University, Beijing China and Faculty of Materials and Chemical Engineering, Yibin University, Yibin
644000, Sichuan China for this study.
Conflicts of Interest: The authors declare no conflict of interest.
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