Dual acidic titania carbocatalyst for cascade reaction of sugar to etherified fuel additives

Article abstract of DOI:10.1016/j.catcom.2018.02.026

An inexpensive carbocatalyst containing Br?nsted acidic sulfonic acid group and Lewis acidic Ti⁴⁺ is found to be effective for cascade conversion of C₆ sugar to 5-ethoxymethylfurfural (EMF) via sequential dehydration, and etherification reactions. HMF and fructose conversions at mild conditions achieved 91% and 64% EMF yields, respectively. The results indicate that the two acid sites interplay synergistically for high EMF yield and minimal ring-opened product ethyl levulinate (EL), another promising biofuel additive. Etherification of 2,5-bis(hydroxymethyl)furan (BHMF) with alcohols of varying carbon lengths formed alkoxymethylfurans (AMF) with high yields. The catalyst retained good activity upon recycling. The nature and strength of the acid sites are elucidated.

Full text of DOI:10.1016/j.catcom.2018.02.026

Accepted Manuscript
Dual acidic titania carbocatalyst for cascade reaction of sugar to
etherified fuel additives
Dinesh Gupta, Basudeb Saha
PII:
S1566-7367(18)30083-9
doi:10.1016/j.catcom.2018.02.026
CATCOM 5340
DOI:
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 December 2017
18 February 2018
28 February 2018
Please cite this article as: Dinesh Gupta, Basudeb Saha , Dual acidic titania carbocatalyst
for cascade reaction of sugar to etherified fuel additives. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2018), doi:10.1016/j.catcom.2018.02.026
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ACCEPTED MANUSCRIPT
Dual Acidic Titania Carbocatalyst for Cascade Reaction of Sugar to
Etherified Fuel Additives
a
b,c
Dinesh Gupta and Basudeb Saha *
a
Catalysis Reaction Engineering, Department of Chemical Engineering, Indian Institute of
Technology Delhi, Delhi, India
b
Catalysis Center for Energy Innovation, University of Delaware, Newark, Delaware 19716,
USA
c
Laboratory of Catalysis, Department of Chemistry, University of Delhi, India
Corresponding author’s email: bsaha@udel.edu

ACCEPTED MANUSCRIPT
Abstract
An inexpensive carbocatalyst containing Brønsted acidic sulfonic acid group and Lewis
4
+
acidic Ti
is found to be effective for cascade conversion of C6 sugar to 5-
ethoxymethylfurfural (EMF) via sequential dehydration, and etherification reactions. HMF
and fructose conversions at mild conditions achieved 91% and 64% EMF yields, respectively.
The results indicate that the two acid sites interplay synergistically for high EMF yield and
minimal ring-opened product ethyl levulinate (EL), another promising biofuel additive.
Etherification of 2,5-bis(hydroxymethyl)furan (BHMF) with alcohols of varying carbon
lengths formed alkoxymethylfurans (AMF) with high yields. The catalyst retained good
activity upon recycling. The nature and strength of the acid sites are elucidated.
Keywords:
Etherification,
Titanium
dioxide,
Lewis
acid,
carbocatalyst,
5-
alkoxymethylfuran, 5-ethoxymethyluran

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1
. Introduction
The awareness of climate change and high market volatility of conventional fuels, owing to
demand-supply imbalance, necessitates to the development of renewable alternatives and
sustainable technologies to meet the growing energy demand for future generations [1]. The
catalytic conversion of non-food biomass or cellulosic sugars into liquid fuels, e.g., 2,5-
dimethylfuran (DMF) [2], 5-ethoxymethylfurtural (EMF) [3, 4] and high carbon alkanes
(diesel, gasoline, jet fuel) [5-7] has been reported. However, poor process economics is the
major challenge towards commercialization. Thus, production of low oxygenated EMF and
analogous alkoxymethylfurans (AMF) via cascade catalysis of cellulosic sugars or
etherification of sugar derived 5-hydroxymethylfurfural (HMF) can be economically more
competitive.
1
1
EMF has a similar energy density (8.7 kWhL ) as standard gasoline (8.8 kWhL )
1
and diesel (9.7 kWhL ) and has superior energy density than ethanol (energy density = 6.1
1
kWhL ) [8, 9]. Its high boiling point (274 C) is another advantage. Avantium has tested
2
0% EMF blended diesel in passenger vehicle engine. They found EMF can reduce SOx
emission, solid particulate contamination, and soot formation [10]. In addition, the engine ran
smoothly for many hours using EMF blended fuel [11]. EMF can also be used as a flavor and
aroma ingredient in wine and beers [12]. Homogeneous and heterogeneous catalytic systems
[13] have been reported for the production of such fuel additives. Homogeneous catalysts,
e.g., mineral acids, inorganic salts [14, 15], ionic liquids [16, 17], organic acids [18, 19], have
offered moderate to high catalytic performance. The major challenge in homogeneous
catalysis is the separation of the catalysts for recycling.
Therefore, solid acid catalysts are promising from economic and environmental
standpoint. Zeolites, sulfonated metal oxides, modified mesoporous silica and ion-exchange
resins including Amberlyst-15 [20-23] have been used for EMF synthesis. Antunes et al. have

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used sulfonated graphene oxide for the etherification of HMF, which produced a mixture of
EMF (42%), ethyl levulinate (EL) (43%) (another high octane fuel additive) and EMF-
diethyl acetal (9%) at 140 C for 24 h [9]. The catalytic systems containing Lewis and
Brønsted acid sites have been reported to be more selective for EMF synthesis. Thus,
3
+
mesoporous silicas containing Lewis acidic Al achieved 68% EMF yield from HMF at 140
ºC for 5 h [24]. A significant amount of non-fuel by-products, e.g., ethyl 4-oxopentanoate and
1
,1-diethoxy ethane (DE) were formed, which posed a separation challenge and products
purification. A one-pot approach of macro-algae derived agar conversion to a mixture of
EMF and EL (5:2) using a SO3H functionalized Dowex 50WX8 catalyst in the presence of
CrCl2 or 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) has been reported, [25], but poor
yield (30%) was the main drawback.
Recently, we have reported the synthesis of a sulfonated carbocatalyst containing
4
+
Brønsted acidic sulfonic acid group and Lewis acidic Ti (hereto referred as Glu-TsOH-Ti)
via a one-pot hydrothermal method [26]. This dual acidic carbocatalyst exhibited excellent
catalytic performance for dehydration of glucose and xylose to HMF and furfural,
respectively [26], consistent with another report in which a hydrothermally prepared
sulfonated carbocatalyst has been reported to be performed better over commercial zeolites,
niobic acid or Amberlyst-15 for cellulosic biofuels production [27]. Usually sulfonated
carbonaceous materials are synthesized by two-step processes involving preparation of
carbonaceous materials in the first step followed by incorporation of –SO3H group under
harsh oxidation conditions using concentrated sulfuric acid [28]. To avoid harsh reaction
conditions, we reported a one-pot method by heating a homogeneous mixture of glucose
(
Glu) and p-toluenesulfonic acid (TsOH) and titanium(IV) isopropoxide in a sealed autoclave
at 180 ºC for 24 h. The ratio of Brønsted to Lewis acid density in the carbocatalyst was 1.2
26]. In addition, the acid sites were more accessible for cascade isomerization and
[

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dehydration of sugars [26]. Herein, we report that the Glu-TsOH-Ti catalyst is effective for
the etherification of HMF and 2,5-bis(hydroxymethyl)furan (BHMF) to their corresponding
etherified fuel additives.
2
. Experimental section
Materials, detailed instrumentation and catalysis methods, and products analysis and
quantification procedures are described in the supporting information. In short, the catalyst
was synthesized and characterized following our reported methods [26]. Dehydration and
etherification reactions were carried out in a 50-mL round bottom flask. NMR spectra of
1
13
various isolated products are shown in the supporting information. This includes H and
C
1
NMR spectra of bis-hydroxymethylfuran (BHMF) (Figures S1-S2), H NMR spectrum of
1
13
isolated EMF (Figure S3), H and C NMR spectra of EMF obtained from fructose (Figures
1
13
1
S4-S5), H and C NMR spectra of 2,5-bis(ethoxymethyl)furan (BEMF; Figure S6 –S7), H
1
3
1
and C NMR spectra of 2,5-bis(methoxymethyl)furan (BMMF; Figures S8 – S9), H NMR
1
of 2,5-bis(propoxymethyl)furan (BPMF; Figure S10),
H
NMR
of 2,5-
2,5-
1
13
bis(butoxymethyl)furan (BBMF; Figure S11), and
bis(isopentoxymethyl)furan (BIPMF; Figure S12-S13).
H and
C spectra of
3
3
.0 Results and Discussions
.1 Material characterization. The dual acidic Glu-TsOH-Ti catalyst was characterized by
powder XRD, FTIR, NH3-TPD, pyridine-FTIR, FE-SEM, HR-TEM, N2-sorption, AAS, and
-
1
-1
ICP-OES techniques [26]. FTIR spectrum showed peaks at =1010 cm , 1035 cm , and
-
1
~
1115 cm for –SO3H groups (Figure S14). Pyridine-FTIR spectrum also confirmed the
presence of Brønsted and Lewis acid sites (Figure S15). The wide-angle XRD pattern showed
three major crystalline peaks at 2= 25.3, 37.8, and 48.08, corresponding to the (101),

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(004) and (200) crystal planes of anatase TiO2 (Figure 1a), (JCPDS File Card No. 21-1272).
2
2
XPS analysis revealed two Ti 2p peaks at 457.8 eV for p3/2 and 463.4 eV for p1/2 (Figure 1b)
29]. Detailed XPS data are in the supporting information (Figure S18). Higher binding
energies for Ti 2p peaks of Glu-TsOH-Ti than that of pure TiO2 lattice could be attributed to
the electronic interaction of Ti with –SO3H; thus, TiO2 in Glu-TsOH-Ti could be
[
considerably modified as reported earlier [30]. TEM and HRTEM (Figures 1c and 1d) images
show plate-like TiO2 particles of dimensions of 10–20 nm that are self-assembled (loosely
assembled). Calculation of d-spacing value (d=0.35 nm) from the HRTEM analysis (Figure
Figure 1: Characterization of Glu-TsOH-Ti: Powder-XRD (a), XPS-spectra of Ti 2p (b)
TEM, HR-TEM (c) and (d). TiO2 particles in the TEM image are indexed.
1
d) suggests that TiO2 exists in anatase form. The HRTEM data also correlated with FESEM
image showing the material is composed of nanoparticles of approximately 20 nm size that

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are aggregated to form larger particles in some part of the image (Figures S16 and S17).
Elemental mapping by energy dispersive X-ray spectroscopy (EDS) in SEM also confirmed
the presence of both titanium and sulfur atoms in the material. Further analysis by AAS
suggested the presence of 3.2 wt% titanium. EDS-SEM data showed that the material
contained 76% and 9.1 wt% C and O, respectively. The BET surface area of Glu-TsOH-Ti is
2
-1
3 -1
4
2.5 m g with a pore volume of 0.0543 cm g (N2 adsorption-desorption plot, Figure S19).
NH3-TPD showed three desorption peaks for ammonia at 194.7 C, 297.9 C and 298.8 C
-
1
with a total acid density of 1.03 mmolg . Total Brønsted acid density, as determined from the
1
amount of sulfur content by ICP-OES method, was measured as 0.56 mmol g- . Thus, the
ratio of Brønsted to Lewis acid density in the catalyst is 1.2.
3
.2 Catalysis
3
.2.1 HMF to EMF. Initial experiments were conducted using pure HMF as the starting
substrate. HMF (1 mmol) etherification with ethanol (5 mL) in the presence of 50 mg catalyst
1
achieved 91% EMF yield at 90 C for 6 h. The H NMR spectrum (Figure S3) showed no
signals for unreacted HMF. A blank experiment without the catalyst yields only 3% EMF,
which confirms the true effectiveness of Glu-TsOH-Ti. A previous report using mesoporous
3
+
silica catalysts containing Lewis acidic Al (Al-MCM-41 (50)) claimed 68% EMF yield
from HMF at 140 ºC for 5 h [31]. Although the total acidity of Al-MCM-41 (50) was not
reported, a comparison of the catalytic performance of Glu-TsOH-Ti and Al-MCM-41 (50)
suggests the superior activity of Glu-TsOH-Ti (91% EMF at 90 ºC) than that of Al-MCM-41
(50) (68% EMF at 140 ºC). The effectiveness of Glu-TsOH-Ti for EMF synthesis was then
compared with other reported heterogeneous catalysts (Ar-SO H-SBA-15, Ar-CMSs-SO H,
3
3
OMC-SO H and Glu-TsOH-Ti) in Table S1. Sulfonated carbon materials and Glu-TsOH-Ti
3
are the most effective, in terms of EMF yield, although a direct comparison of these two

ACCEPTED MANUSCRIPT
catalysts is difficult because of differences in the reaction conditions. Higher activity of Glu-
TsOH-Ti in our earlier publication [26], is attributed to better accessibility of its acid sites
3
+
and dual acidity, consistent with a prior report claiming Lewis acidic Al in extra-framework
zeolite facilitated HMF etherification to EMF [24]. It was proposed that HMF forms an
oxonium by the Lewis acid sites upon adsorption on the surface, which then reacts with
ethanol.
The reaction profile (Figure 2) reveals the formation of the ring-opening product, EL,
upon conducting the reaction for a prolonged time. Because of ring-opening of EMF, its yield
decreased and EL formed. The ring-opening of EMF is likely occurring by the Brønsted acid
-
1
sites (0.56 mmol g ). A prior report explained that Brønsted acidic catalysts (H2SO4,
®
Amberlyst and Al-MCM-41 (25) favored the ring-rehydration and etherification, while the
3
+
Lewis acid sites in the catalysts (ZrO2-SBA-15, Al isolated sites in extra-framework Al-
MCM-41 (50)) acted synergistically to form EMF selectively.[24] Another paper reported
2
-
that a SO4 /TiO2 catalyst exhibited better catalytic performance and selectivity to methyl
levulinate[32] from glucose when compared the results with TiO2, Zr3(PO4)4, H-mordenite
2
-
and ZSM-5 catalysts. It is likely due to the higher acid density of SO4 /TiO2 (0.63 mmol/g)
2
-
than those of SO4 /ZrO2 (0.57 mmol/g) and Zr3(PO4)4 (0.30 mmol/g) [32]. The Lewis acidity
4
+
2-
of Ti in SO /TiO also likely facilitated isomerization of glucose to fructose, a key step to
4
2
form HMF and then EMF.

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1
00
HMF
EMF
EL
EMF
8
6
4
2
0
0
0
0
0
HMF
EL
0
5
10
15
20
25
30
35
Time (hr)
Figure 2. The reaction profile of HMF etherification with the Glu-TsOH-Ti catalyst at 90 ºC.
3
.2.2 BHMF to 2,5-Bis(alkoxymethyl)furan. The etherification of BHMF was conducted
with ethanol, methanol, n-propanol, n-butanol, and isoamyl alcohol to synthesize the
corresponding AMF products (Table 1). The reaction temperatures with different alcohols
were adjusted to keep the temperatures before their boiling points; however, it didn’t change
the yield of the corresponding etherified products significantly. A reaction with high catalyst
loading showed a little effect on the yield (entry 5). The reaction time was found to have a
profound effect on the yield and the nature of the etherified product. It was found that –OH
groups of BHMF etherified with ethanol. First, 5-(ethoxymethyl)furan alcohol (EMFA) and
BEMF are formed in 4 h (entry 2). When the reaction continued for 8 h, EMFA is fully
converted to BEMF as the sole product with 92% yield and 100% selectivity (entry 3). The
etherification of BHMF with ethanol was conducted using varying amounts of Glu-TsOH-Ti
(Figure S20). It shows the catalytic performance enhanced with increasing the catalyst
amount from 5 mg to 20 mg. Higher loadings (>20 mg) showed no pronounced effect, which
is likely due to the mass transport limitation of the catalyst.

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Table 1. Etherification of BHMF with different alcohols.
Yield
C) (%)
T
Entry Alcohols
Etherified products
(
1
MeOH
EtOH
70
89
a
2
80
51
3
4
5
6
EtOH
80
92
90
93
88
nPrOH
nBuOH
100
120
b
Isoamyl 120
Reaction conditions: BHMF = 1 mmol; alcohols = 5 mL, Glu-TsOH-Ti =20 mg and 8 h
a
b
reaction time. reaction time = 4 h; Glu-TsOH-Ti = 40 mg.
3
.2.3 Fructose to HMF. The effectiveness of the Glu-TsOH-Ti catalyst was further
evaluated for one-pot conversion of fructose to EMF, where dehydration and etherification
take place sequentially. A preliminary reaction yielded 60% HMF, 29% EMF and 6% EL for
6
h at the quantitative conversion of fructose (entry 1, Table 2). The yield of EMF improved
to 64% upon conducting the reaction for a prolonged time (24 h; entry 2, Table 2). EL (13%)
and HMF (21%) accounted for the remaining carbon balance. Ring-opening of EMF to EL
was observed for a 48 h reaction (entries 3 and 4, Table 2). The color of the product solutions
turned darker for carrying out the reaction for longer times, indicating some amount of

ACCEPTED MANUSCRIPT
humins could be formed [26]. A blank experiment without the catalyst showed no fructose
conversion. Another reaction using glucose as the starting substrate gave low EMF and EL
yields (entry 5, Table 2). A prior study reported that a sulfonated silica-carbon catalyst was
effective for the conversion of fructose to EL and EMF, giving up to 80% total yield [13].
However, this catalyst was ineffective for glucose isomerization to fructose, resulting in poor
yields of EL (7%) and EMF (4%). A similar observation was noted in another report for
glucose conversion to EL/EMF using a sulfonic acid functionalized carbocatalyst [33]. In
contrast, our carbocatalyst gives a slightly higher yield of EMF in comparison with the results
of the sulfonated silica-carbon catalyst [13], due to high HMF yield via glucose isomerization
and dehydration by the Lewis and Brønsted acid sites, as noted in our previous study [26].
HMF is then etherified to EMF by the Bronsted acid sites. The ring-opening of EMF occurs
by the Brønsted sites when the reaction is prolonged.
Table 2. Fructose or glucose conversions to etherified products.
Fructose or
glucose
Conv. (%)
Yield (mol%)
HMF EMF EL
Entries Time (h)
Glu-TSOH-Ti
mg)
(
1
2
3
4
5
6
50
50
100
100
100
100
nd
60
21
nd
nd
24
nd
26
24
25
29
64
66
43
19
17
61
59
58
6
13
18
36
7
24
30
48
24
24
24
24
24
50
50
25
a
6
50
87
4
7
8
9
cycle 2
cycle 5
cycle 6
100
100
100
10
9
7
a
Other reaction conditions: Ethanol = 5 mL, temperature = 120 °C. glucose as the starting
substrate. Entries 7-9 show the catalyst recyclability results. Nd = not determined.

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The catalyst recyclability studies show (entries 7-9, Table 2) a slight decrease in the
th
etherified and ring-opened products in the 6 cycle, which could be due to the loss of the
catalyst mass during recovery. The hot-filtration test of the catalyst for HMF etherification
indicates no leaching of its Brønsted acid sites (Figure S21).
4
.0 Conclusions
We reported a dual acidic carbocatalyst, containing titania as Lewis acid and sulfonated
groups as Bronsted acid sites, is effective for HMF etherification to high octane fuel additives
with maximum yield of up to 93%. The Lewis and Brønsted acid sites act synergistically to
enable higher yield of furan ring containing products and minimal ring-opened side products.
The two acid sites also enhanced the catalyst’s performance and products selectivity in one-
pot conversion of fructose via sequential dehydration and etherification reactions.
5
.0 Acknowledgements
DG is thankful to SERB-DST for a National Postdoctoral Fellowship (NPDF; award number
PDF/2016/000458), Government of India. BS also acknowledges the financial support from
the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under
Award number DE-SC0001004 for the later stage of the manuscript.

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Graphical abstract

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Highlight

HMF etherified fuel additives of different carbon lengths are synthesized with high
yields


Furan rings retained in the etherified products with minimal ring-opening
A dual acidic carbocatalyst acts synergistically to achieve products with high
selectivity

A cascade of conversion of fructose to etherified products is achieved