Organic radical functionalized SBA-15 as a heterogeneous catalyst for facile oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

Article abstract of DOI:10.1016/j.molcata.2015.04.008

Organic radical functionalized mesoporous silica was prepared by the immobilization of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on ordered mesoporous silica (SBA-15) and applied as a heterogeneous catalyst (TEMPO-SBA-15) for the synthesis of 2,5-diformylfuran (2,5-DFF) from 5-hydroxymethylfurfural (5-HMF). A 73 % 2,5-DFF yield was obtained from 5-HMF oxidation under mild reaction conditions in ethyl acetate with the addition of [bis(acetoxy) iodo]benzene (BAIB) and acetic acid as co-oxidants in an oxygen-rich environment. The presence of BAIB and oxygen facilitated the in-situ regeneration of TEMPO (the hydroxylamine form) whereas acetic acid addition aided in the initial formation of oxoammonium form of TEMPO, which was responsible for facile 5-HMF oxidation. All reaction components were found critical in maximizing the 2,5-DFF yield. The pore size distribution and architecture of SBA-15 played essential roles for the diffusive transport of reactants and products and resulted in improved reaction selectivity. Aside from these advantages, TEMPO-SBA-15 catalyst was easily separated and re-used several times with negligible loss in its catalytic activity. The prepared heterogeneous TEMPO-SBA-15 is an efficient catalyst that offers a sustainable route for 2,5-DFF production.

Full text of DOI:10.1016/j.molcata.2015.04.008

Accepted Manuscript
Title: Organic Radical Functionalized SBA-15 as a
Heterogeneous Catalyst for Facile Oxidation of
5-Hydroxymethylfurfural to 2,5-Diformylfuran
Author: Neha Mittal Grace M. Nisola Jeong Gil Seo
Seong-Poong Lee Wook-Jin Chung
PII:
S1381-1169(15)00147-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.04.008
MOLCAA 9471
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
8-8-2014
24-3-2015
13-4-2015
Please cite this article as: Neha Mittal, Grace M.Nisola, Jeong Gil Seo, Seong-
Poong Lee, Wook-Jin Chung, Organic Radical Functionalized SBA-15 as
a Heterogeneous Catalyst for Facile Oxidation of 5-Hydroxymethylfurfural
to 2,5-Diformylfuran, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.04.008
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

1
2
Journal of Molecular Catalysis A: Chemical
3
4
Organic Radical FunctionalizedSBA-15 asa Heterogeneous Catalyst for
FacileOxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran
5
6
7
Neha Mittal, Grace M. Nisola, Jeong Gil Seo*, Seong-PoongLee, Wook-Jin Chung
8
9
Energy and Environment Fusion Technology Center (E2FTC),
Department of Energy and Biotechnology (DEB), Myongji University,
10
Myongji-ro 116, Namdong,Cheoingu, Yongin-city, Gyeonggi-Do, 449-728, Republic of Korea
11
12
13
14
15
16
17
18
*Corresponding author
(E-Mail:jgseo@mju.ac.kr, Tel: +82-31-324-1338, Fax: +82-31-336-6336)
1

19
20
2

21 Research Highlights
22
23
24
25
26
1. TEMPO-SBA-15 was prepared via silanization of organosilane-functionalized TEMPO.
2. Highly selective 5-HMF oxidation to 2,5-DFF over heterogeneous TEMPO-SBA-15.
3. TEMPO-SBA-15 exhibited excellent recyclability without loss in catalytic activity.
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
Abstract
Organic radical functionalized mesoporous silicawas prepared by theimmobilization of
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on ordered mesoporous silica (SBA-15)and
applied as a heterogeneouscatalyst (TEMPO-SBA-15) for the synthesis of 2,5-diformylfuran
(2,5-DFF)from 5-hydroxymethylfurfural (5-HMF). A73% 2,5-DFF yield was obtained from 5-
HMF oxidation under mild reaction conditions in ethyl acetate with the addition of
[Bis(acetoxy)iodo]benzene (BAIB) and acetic acidas co-oxidants in an oxygen-rich
environment.The presence of BAIB and oxygen facilitated the in-situ regeneration of TEMPO
(the hydroxylamine form) whereas acetic acid additionaided in the initial formation of
oxoammonium form of TEMPO, which was responsible for facile5-HMF oxidation. All reaction
components were found critical in maximizing the 2,5-DFF yield. The pore size distribution and
architecture of SBA-15 played essential roles for the diffusive transport of reactants and products
and resulted in improved reaction selectivity.Aside from these advantages, TEMPO-SBA-
15catalyst was easily separated and re-usedseveral times with negligibleloss in its catalytic
activity.The prepared heterogeneous TEMPO-SBA-15 is an efficient catalyst that offers a
sustainable route for 2,5-DFF production.
3

45
46
Keywords: 2,5-Diformylfuran, 5-Hydroxymethylfurfural, 4-Hydroxy-2,2,6,6-
Tetramethylpiperidine-1-Oxyl (TEMPO), Oxidation, Immobilization, Mesoporous Silica
4

47
1. Introduction
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
Furan derivatives are value-added chemicals that can be derived from biomass feedstocks.
Particularly, 2,5-diformylfuran (2,5-DFF) is a commercially important precursor for the
synthesis of various pharmaceuticals, fungicides, ligands of various poly-Schiff bases,
polymers,and cross-linking agents[1-4]. 2,5-DFF is typically synthesized by selective oxidation
of 5-hydroxymethylfurfural (5-HMF)[5], a dehydration product of biomass-derived hexoses [5-
7].
Many reports have been accounted for the oxidation of 5-HMF using conventional, metal
containinghomogeneous catalysts like Pb(OAc)4-pyridine, pyridiniumchlorochromate, K2Cr2O7-
DMSO,Co/Mn acetates,V2O5, andCu(NO3)2/VOSO4 [8-13]. Althoughadequate 2,5-DFF yields
have been reported, most of thesecatalysts are laden with toxic metals. Moreover, many of the
reported DFF syntheses are conductedundersevere reaction conditions(≥80oC and ≥10 bar) with
several hours of reaction time. Thisincreases the cost of production set-up and in few cases
leadsto side products formation byover-oxidation, decarbonylation,and cross-polymerizationof 5-
HMF [13].Parallel to the need of cheap, easily handled and less toxic catalytic system, a re-
usable oxidation catalystis also highly desirable to ease environmental and economicburdens.
Several heterogeneous catalytic systems likewise have been employedfor the oxidation of 5-
HMF such asvanadyl phosphate[14], vanadyl acetylacetonate immobilized on poly(4-
vinylpyridine) polymer[15], Ru supported over hydrotalcite [16], porous manganese oxide [17],
zeolite-supported vanadium pentoxide[18] and silver impregnated manganese oxide [19].
Despite the remarkable developments in2,5-DFF synthesis with fair yield, nearly all these
systems are based on transition metal catalystsand high boiling point organic solventswhich
hinder the facile product separation[14-19]. Moreover, the active sites in these catalysts were
5

70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
transition metals, requiring higher activation energy to perform and thus these systems were
applied at higher reaction temperature.
In context of a metal-free catalyst, nitroxyl based radicals are being explored due to
theirefficiency inoxidizing various types of alcohols[20-22].In addition, these have shown less
toxicity and remarkable reversible redox behavior[21-24].One of these stable organic radicals,
2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) and its derivatives have been used for 5-HMF
oxidation in the presence of various co-oxidants[25-27]. Particularly, an earlier work
successfully produced 2,5-DFF under mild reaction conditions viahomogeneous TEMPO-
catalyzed 5-HMF oxidation with co-oxidantsBis(acetoxy)iodo]benzene (BAIB) and acetic acid
(AcOH) [27]. Despite the satisfactory formation of 2,5-DFF, this mild homogeneous reaction
could not address the catalyst recyclability and high reaction selectivity due to formation of side
product.Thus, the TEMPO moiety immobilized on an appropriate solid supportcan provide
aneffective, sustainable,and metal free catalytic system. Several supported TEMPO catalyst
systems like on silica and ferrite surface[28], on carbon nanotubes [29] and mesoporous silica
[30] havebeen reported for the oxidation of alcohols.However, these catalyst systems were
comprised of either transition metals[28] or toxic and hazardous reagents like tert-butyl nitrite
(TBN)[30].Recently oxidation of 5-HMF by TEMPO supported on silica coated iron oxide
nanoparticles was demonstrated [31]. But the use ofnocuous TBN and high boiling point solvent
toluene with hours long reaction time makes this methodology inappropriate from the viewpoint
of green and sustainable chemistry. Therefore, our continuous interest is in developing a less
toxic reaction system for 5-HMF oxidation in low boiling point solvents [27].Among various
support materials for catalysts, the suitability of mesoporous silica like SBA-15 has been
demonstrated in numerous studies, as the silanol groups on their surfaces are easy to
6

93
94
functionalize[32].The stability of TEMPO can be assured by its covalent immobilization on the
supportthroughorganosilane linkers. The unique structural properties of mesoporous SBA-15 like
a high specific surface area due to its well-ordered porous structure, large pore volumes,and
narrow pore size distribution (Fig. 1B, inset), would allow a high degree of TEMPO
functionalization leading towards anefficient heterogeneous TEMPO catalyst.
Thus, herein, TEMPO-functionalized-SBA-15 (TEMPO-SBA-15)catalyst was prepared
through silanization of TEMPO radicals functionalized with an organosilane linker.
Morphological and structural properties of bare SBA-15 and its functionalized form (TEMPO-
SBA-15) were examined by X-ray diffraction (XRD), N2 adsorption-desorption,
thermogravimetric analysis (TGA), FT-IR,and scanning electron microscopy (SEM).
Comparisons in their characterization results were performed to confirm the successful
immobilization of TEMPO on SBA-15. The prepared TEMPO-SBA-15was applied as
heterogeneous catalyst for the oxidation of 5-HMF to 2,5-DFF under mild reaction conditions.
For maximum 2,5-DFF yields, effects of co-oxidantswere also determined. Lastly, the long-term
use of TEMPO-SBA-15 was also inspected through a series of repeated 5-HMF oxidation
reactions using recycled catalyst.
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
7

110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
2. Experimental
2.1.Materials
4-hydroxy-TEMPO (97%), BAIB (98%), organosilane linker (3-chloropropyl)-
triethoxysilane (CPTES) (95%), Poly(ethyleneglycol)-block-poly(propyleneglycol)-block-
poly(ethyleneglycol) (Pluronic®P123),silica (fumed, surface area 390 m2/g) and dry toluene
(99.8%) were obtained from Sigma-Aldrich (South Korea). 5-hydroxymethylfufural (5-
HMF)(98%), tetraethylorthosilicate (TEOS) (98%), sodium hydride (NaH) (60% dispersion in
oil), and dry tetrahydrofuran (THF) (99.8%) were purchased from Acros Organics while 2,5-
diformylfuran (2,5-DFF) (>98%) was procured from Tokyo Chemicals (Japan). Hydrochloric
acid (HCl) (35-37%) was bought from Showa Chemicals whereas other solvents including
diethyl ether (99%) and ethyl acetate (99.8%) were obtained from Dae Jung Chemicals (South
Korea). Sodium sulfate (99%) was purchased from Junsei Chemicals. All chemicals were of
reagent grade or higher, and were directly used without further purification.
2.2. Preparation of catalyst
2.2.1. Preparation of SBA-15
SBA-15 was synthesizedaccording to a modified method reported elsewhere using tri-
block copolymer Pluronic®P123 (Average Mol. Weight = 5800) as a structure directing
agent[33]. A 10 g sample of P123 was initially dissolved ina mixture of 325 mL of deionized
(DI) water and 60 mL of HCl (35.0-37.0 %) followed by TEOS(20.6 g) addition, with 800
rpmagitation,at 40 oC for 2h.The cloudy dispersion was then statically aged at 40 oC for 24 h and
then at 80 oC for another 24 h.The solid product was filtered off;repeatedlywashed with hot (80
oC) deionized (DI)water for the removal of residual reactants. After air-drying at room
8

133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
temperature for 4 days, the dried powder was calcinedat 500oC (in air,5 h)with2.5 oC min-
1heating rate from room temperature.
2.2.2. Synthesis of 3-(4-oxy-TEMPO)propyl-triethoxysilane
The reactions were carried out in nitrogen (N2) atmosphere. Sodium hydride (0.2758 g, 6.9
mmol) was suspended in dry THF (20 mL) cooledin an ice-bath. A separately prepared 4-
hydroxy-TEMPO (1.0 g, 5.8 mmol)solution in 5 mLdry THF was added dropwise in the
coldNaHslurry. The mixture was brought to room temperature (RT) and stirred to facilitate the
reaction. Aftercomplete gas evolution, the mixture was re-cooled at 0oC and CPTES (1.6615 g,
6.9 mmol) was addeddropwise.The reaction was continued at room temperature for another 48 h
(Scheme 1).The crude product wasretrieved by solvent evaporation under reduced pressure. To
precipitate the impurities, the oily product was dissolved in diethyl ether. The filtered diethyl
ether was dried over sodium sulfateand then re-filtered. The solvent wasremovedina
rotaryevaporator to obtain the pure dark oily orange 3-(4-oxy-TEMPO)propyl-
triethoxysilane(denoted as PTES-TEMPO).
2.2.3. Preparation of TEMPO-SBA-15 catalyst
Calcined SBA-15 (2.0 g) was vacuum-dried for 1 h at 90oC and then suspended in 90
mLdry toluene with vigorous stirring under N2atmosphere.A solution of PTES-TEMPO (1.53
mmol) in toluene was then added dropwise in the SBA-15 suspension.The mixture was refluxed
at 110 oCfor20 h with continuous stirring (Scheme 1). After which, the reaction was terminated
and cooled to room temperature.The product, denoted as TEMPO-SBA-15, was filtered, washed
three times with toluene, and then dried in air at room temperature.
9

157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
2.3. Catalyst characterization
Powder low angle X-ray diffraction (XRD) patterns of SBA-15 and TEMPO-SBA-15
samples were acquired usingPANalyticalEMPYREAN SWAX diffractometer(40 kV,25 mA)
withNi-filtered Cu-K radiation (λ = 0.15406 nm)source scanned at 2angles from 0.3o to
10o.The FT-IRspectra of the samples were recorded by a Shimadzu IRPrestige-21
spectrophotometer equipped with DLaTGS detector using standard KBr disc method (1 wt.%
sample) between wavenumbers 400 and 4000 cm-1 at room temperature. Thermogravimetric
analysis (TGA) was carried out ina SCINCO TGA (N-1000)from 35oCto 1000oCwith a ramping
rate of 5 oC min−1, supplied with 50 mLmin-1 ofN2. N2 adsorption-desorption isothermswere
obtained using a Micromeritics ASAP-2020 surface area analyzer at 195.8 oC. The sampleswere
outgassed(in vacuum)at 100 oC for 24 h prior to the adsorption experiments. The carbon and
nitrogen contents of the samples were determined by elementanalysis using Vario EL cube
CHNS analyzer (Elementar Corporation, Germany). The morphology of the samples was
observed under Hitachi S-3500N Scanning Electron Microscope (SEM) and JEOL JSM-7600F
Field Emission Scanning Electron Microscope(FE-SEM).
2.4. Catalyst activity
2.4.1. Typical procedure for the synthesis of 2,5-DFF from 5-HMF
All experiments were carried out in two-necked round bottom flasks initially purged with
O2 and then fastenedwith an O2-filled balloon (1 atm).The flasks were placed in a temperature-
controlled oil bath with magnetic stirring.Typically, appropriate amounts of the reactants 5-
HMF, TEMPO-SBA-15, BAIB,and acetic acid (if used) were allowed to reactin ethyl
acetatesolvent (10-15 mL). The reaction mixture was magneticallystirred fora certain reaction
10

180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
time, at a controlledtemperature.After each run, the reaction mixture was filtered to separate the
solid catalyst. The filtrate was dried under reduced pressure,dissolved in deionized(DI) water and
then analyzedinhigh pressure liquid chromatography (HPLC).
2.5. Product analysis and quantification
Prior to HPLC analysis, samples were filtered through a syringe filter (0.2 m Nylon).The
Waters HPLCwas equipped with Waters 2414 refractive index (RI) detector maintained at 40 oC
and BioradAminex HPX-87H ion-exclusion column (300×7.8 mm) kept at 60oC.A 5 mM H2SO4
mobile phase was deliveredat a flow rate of 0.6 mLmin-1.The HPLC retention times for reaction
components were observed as 14.7 min for AcOH, 14.7 min for 2,5-FDCA, 28.0 min for 5-HMF
and 32.7 min for 2,5-DFF. The concentrations of 2,5-DFF (i.e. [DFF]) and 5-HMF (i.e. [HMF]
weredetermined against calibration curves of standard samples.The amount of produced 2,5-DFF
(WDFF)and corresponding reaction yields werecalculated according to equations (1) and (2). 2,5-
DFF selectivity was calculated according to equation S1 (Eq. S1). V is the sample volume,
MWis the molecular weight of 5-HMF(126.11 g/mole)or 2,5-DFF (124.09 g/mole)while WRM,
WAL, and WHMF arethe masses of the entire reaction mixture, aliquot for analysis and supplied
HMF, respectively. All reported data are average values of triplicate reaction runs.
[DFF]  V(mL) W (mg)
RM
W
(mg) 
DFF
197
(1)
(2)
W
(mg)
AL












W
MW
HMF
MW
DFF
DFF
DFF yield(%) 
100
198
W
HMF
199
200
2.6. Recycling of the catalyst
11

201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
After oxidation reaction, TEMPO-SBA-15was retrieved from the reaction mixture via
filtration. The used catalystwaswashed with the reaction solvent to remove traces of reaction
debris.Then the catalyst wasair-dried at room temperatureprior toreuse.
3. Results and discussion
3.1. Synthesisof catalyst
The mesoporous silica support SBA-15 was prepared throughTEOShydrolysis in the
presence of the triblock co-polymer P123 (EO20PO70EO20) as the surfactant [33]. Meanwhile
in a separate reaction, TEMPO was etherified with CPTES to form PTES-TEMPO. To complete
the synthesis of TEMPO-SBA-15, PTES-TEMPO was used to silanizethe support, in which
covalent Si-O-Si bonds were formed by the reaction of silanol groups on SBA-15 with the
ethoxy groups of PTES-TEMPO (Scheme 1).
<<Insert Scheme 1>>
3.2. Characterization of catalyst
Textural properties of SBA-15 were analyzed by X-ray diffraction (XRD) pattern and N2
adsorption-desorption isotherms (Fig. 1). The bare support material (SBA-15) exhibited three
well-resolved diffraction peaks at 2θ = 0.9°, 1.5°, and 1.7°in the XRD pattern (Fig. 1A),which
correspond toreflection planes of(100), (110), and (200),respectively. These peaks indicate a
significant degree of long-range order and a well-formed two-dimensional (2-D) hexagonal
meso-structure ofSBA-15 [34].Afterimmobilization, the resultingTEMPO-SBA-15 exhibited all
of the prominentdiffraction peaks at 2θ= 0.9°, 1.5°, and 1.7° (Fig. 1A) similar to SBA-15. This
implies that the 2-D hexagonal meso-structure of SBA-15 does not deteriorate after the
12

224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
immobilization of PTES-TEMPO. Thus,preparedTEMPO-SBA-15 demonstrated analogous
reflection planes to its support material,suggesting that the periodic mesoporousstructure was
retained [35].Furthermore, the decrease in intensity of diffraction peak corresponding to
reflection plane of (100)confirms the successfulimmobilization of PTES-TEMPO to SBA-15
surface.
<<Insert Figure 1>>
The N2adsorption-desorption isotherms obtained from bare SBA-15 exhibits the typical
type IV isotherm pattern having sharp capillary condensation with H1 hysteresis loop at higher
relative pressure (Fig. 1B). This pertains tothe 2-D hexagonal symmetry of a mesoporous
material [36-37]. TEMPO-SBA-15 showed similar adsorption-desorption isotherm curves and
hysteresis loop with that of bare SBA-15.In addition, both SBA-15 and TEMPO-SBA-15 showed
narrow pore size distributions(Fig. 1B, inset), indicating structural consistency of the catalyst
before and after immobilization.The bare SBA-15 showed a very high BET surface area of 883
m2/g (Table 1)and large total pore volume of 1.24 cm3/g (Table 1).The gradual decrease in BET
surface area and total pore volume upon immobilization of PTES-TEMPO confirms its
successful immobilization to the pore channels of SBA-15. Nonetheless, incorporation of
TEMPO on SBA-15 did not reduce the average pore size of the supportdue to simultaneous
functionalization in micro- and meso-pores which counterbalances the changes in pore size.
Thus, kept intact the mesoporous structure of the support material even after functionalization.
<<Insert Table 1>>
13

246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
The TG analyses of the heterogeneous catalyst and SBA-15 were done to assess the
functionalization of TEMPO on SBA-15 and to estimate the loading of TEMPO radical (Fig. 2).
The first weight losses(< 2%) in bare SBA-15 and TEMPO-SBA-15 were observed below100
oC, due to the evaporation of surface-adsorbed moisture. Further weight loss up to 8.58% was
observed in TEMPO-SBA-15from 110to 235 oCwith a sharp decrease around156 oC, dueto the
degradation of the3-(4-oxy-TEMPO)propyl chain on the SBA-15 surface[38].This result
indicated the loss of 0.4 mmol of 3-(4-oxy-TEMPO)propyl chainper gram of the catalyst
material,from which,0.3 mmol/g loading ofTEMPOwas estimated. Without the functional groups
on the bare support,such weight loss was absent in SBA-15 within the same temperature range.
Only 3.21%total weight loss was observed for bare SBA-15,mainly due to the condensation of
surface silanolsthatformssiloxane groups and releaseof watermolecules at higher temperatures (>
600oC)[39]. For TEMPO-SBA-15, the significant weight loss of 16.68% between 235 and 1000
°C is associated to the breakdown of the unreactedethoxy groups of the organosilane and
partially to the silanol condensation. Elementalanalysis also determined the loading of TEMPO
as0.3 mmol/g of TEMPO-SBA-15 (Table 1) along with 15.7 C/N ratio, which strongly
corroborates the TGA result.
<<Insert Figure 2>>
The successful immobilization of the TEMPOon SBA-15 was furtherconfirmed via the FT-
IR spectra(Fig. 3). The IR spectrum ofPTES-TEMPO (Fig. 3a) featurescharacteristic absorption
bands at 1373 and 1165 cm-1for the C–N and C–O stretching vibrations, respectively[29].
Additionally, the spectrum presents peaks in the range of 2995-2870 cm-1associated with C–H
stretching vibrations.Characteristic bands of the silica frameworkswere observed both in bare
14

269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
SBA-15 and TEMPO-SBA-15(Fig. 3b and 3c); at 3600-3200 cm-1for stretching of H-bonded
silanol groups, at 1085 and 806 cm-1for asymmetric and symmetric Si–O–Si stretching and at
964 cm-1for Si–O stretching[40]. The absorption peak at 1629 cm-1is associated with the bending
vibration of the adsorbed water[41]. In the heterogeneous catalyst, aC–N stretching vibration was
present (Fig. 3c) at 1377 cm-1, which asserts the presence of TEMPO moiety.The wide band of
Si–O–Si stretching(albeit it overlapped the C-O stretching) concomitant with the decrease in the
peak intensities of (1)O–H stretching and bending (3600-3200 cm-1and 1629 cm-1, respectively)
and (2)Si–O stretching (964 cm-1)are all indications of asuccessful silanization of PTES-TEMPO
on SBA-15.
<<Insert Figure 3>>
The morphologies of bare SBA-15 and TEMPO-SBA-15were examined by SEM asshown
in Fig. 4. The mesoporous SBA-15 features a long-range, self-organized morphology and
uniform-sized particles which were preserved after PTES-TEMPO immobilization (Fig. 4a-4b
and 4c-4d). Both samples were composed of rod like, bean-shaped particles of approximately
800-1000 nm in length. The FE-SEM imagesof SBA-15 and TEMPO-SBA-15 (Fig. 4c and 4d,
respectively) (100 nm scale) also showed well-ordered arrays of mesoporous channels [42] and
intact morphology of SBA-15 after immobilization of TEMPO.
<<Insert Figure 4>>
3.3.Homogeneous TEMPO-catalyzed 5-HMF oxidation
In an earlier work, homogeneous TEMPO-mediated oxidation of 5-HMF in ethyl acetate
afforded 2,5-DFFthrough the ‘oxoammonium’ mechanism [18]. Concomitant with 5-HMF
15

292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
oxidation into 2,5-DFF is the in-situ reduction of the oxoammonium form of TEMPO into
hydroxylamine form. As a regenerant, co-oxidant BAIBis added to oxidize hydroxylamine back
to TEMPO radical form and it also produces acetic acid for the initial dismutation of TEMPO
radical to its catalytically active oxoammoinum form [27]. Thus, 2,5-DFF production in catalytic
TEMPO dosage was possible in the presence of BAIB. For enhanced 2,5-DFF yield, initial
investigationwas performed in a homogeneous system of5-HMF(0.5 mmol)and BAIB(0.75
mmol)with varied TEMPO dosages. Production of 2,5-DFF (Fig. 5) increased with the catalyst
dosage andthe highest valueswere obtained at over 0.075 mmolof TEMPO. The high conversion
of 5-HMF at lower TEMPO dosages is the result of ligand exchange phenomenon with BAIB
(Fig. S4). Due to inadequate TEMPO, yield of 2,5-DFF remained less. Nonetheless, only
moderate 2,5-DFF yield (57.4% maximum at 0.1 mmol TEMPO dosage)was achieved in
homogeneous TEMPO-catalyzed system. Meanwhile, homogeneous TEMPO-catalyzed
reactionsin oxygen-richcondition afforded much higher 2,5-DFF yields and selectivities at entire
range of TEMPO dosagesthan the system in the absence of oxygen (Fig. 5 and Fig.
S3).Molecular oxygen (O2) functions similarly as BAIB by oxidizing the hydroxylamine form of
TEMPO catalyst to its radical formas demonstrated by Sheldon et al [43](Fig. S4).Furthermore,
the effect of oxygen and phenomenon of ligand exchange involving 5-HMF and BAIB were
validated by only 9.8% 5-HMF conversion and 5.2 % 2,5-DFF yield by the BAIB-free system
with oxygen supply (Fig. 7). Additionally, despite the high 2,5-DFF yield in O2-rich
homogeneous system, a minor amount of 2,5-furandicarboxylic acid (2,5-FDCA), an over-
oxidation product of 5-HMF[19], was also observed (Fig. S1).
<<Insert Figure 5>>
16

315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
3.4.5-HMF oxidation in heterogeneous TEMPO-SBA-15 catalyst
With enhanced2,5 DFF productions in O2-rich TEMPO-catalyzed homogeneous system,
heterogeneous reactions were likewise performedover TEMPO-SBA-15 catalyst(Scheme 2).
Highest yield of 52% 2,5-DFF was achieved(Fig. 5) with heterogeneous TEMPO(at 0.075 mmol
of TEMPO) (Fig. S4). Unlike homogeneous system, no 2,5-FDCA was detected in
heterogeneous TEMPO-SBA-15 catalyst system(Fig. S2) resulting in enhanced reaction
selectivity by controlled conversion of 5-HMF to dialdehyde (Fig. S3). In the context of slightly
declined 2,5-DFF production and no 2,5-FDCA formation by TEMPO-SBA-15 (i.e. compared to
homogeneous TEMPO-catalyzed reaction),immobilization of TEMPO moieties into the porous
channels of SBA-15 played a crucial role. These results suggest the critical role of SBA-15 as
TEMPO support.The highly ordered and uniform porous structure of SBA-15 leads to form
uniform active sites of the TEMPO catalyst and also controls the spatial effects. These factors are
crucial for the diffusive transport of reactants to and products from active sites and, thereby
controlling the over-oxidation of product.
<<Insert Scheme 2>>
3.5.Influence of reaction temperature on TEMPO-SBA-15performance
The effect of reaction temperature was also investigated to attain better 2,5-DFF
production. As the reaction componentshave reduced diffusive transport in heterogeneous
catalyst systems, heating the reaction could increase their kinetic mobility and improve their
contact frequency with active sites of catalyst. This effect became modestly observablewhen the
temperature was slightly raised to 40oC as the maximum 2,5-DFF yield improved to 63% (Fig. 6)
from52% (at room temperature) with complete 5-HMF conversion (Table S1) in 60
17

338
339
340
min.Additionally, oxidation of 5-HMF followed consecutive conversion into two intermediate
states; first by rapid ligand exchange with BAIB (Fig. S4 (a)) and then, reaction with
oxoammonium salt (+N〓O) form of TEMPO radical moiety (Fig S4 (b)). The formation of 2,5-
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
DFF occurred simultaneously with the reduction of oxoammonium salt to hydroxylamine form.
Therefore, the production of 2,5-DFF was lagging behind as compared to the quick consumption
of 5-HMF. As a consequence, high 5-HMF conversion with low 2,5-DFF selectivity was
observed in initial stages of the reaction at low reaction temperature.While the rate of 2,5-DFF
formation increased with reaction temperature, the declines in 2,5-DFF yields were observed
under prolonged reaction periods at all temperatures. This is attributable to eventual degradation
of 2,5-DFF to other undetected compounds [11]. Though the maximum 2,5-DFF formation
(57%)at 50 oCwas quickly reached within 30-45 min, the reduced 2,5-DFF selectivity (Table S1)
might have been due to the additional occurrence of 5-HMF degradationat elevated temperatures
[44].
In a previous work, the uninhibited exposure of homogeneous TEMPO to the substrate
resulted in the higher propensity of the reaction components to degradation. Increase in
temperature beyond ambient condition was detrimental to selective 2,5-DFF production [27]. In
the current study, however, the insulating effect of SBA-15 on TEMPO was beneficial as it
rendered TEMPO-SBA-15to perform more efficiently in a wider temperature range. From the
results, reaction at 40oC for 60 min was most appropriate to enhance 2,5-DFF yield with minimal
degradation of reaction components.
<<Insert Figure 6>>
3.6.Acetic acid addition for enhanced TEMPO-SBA-15 performance
18

361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
To initiate 5-HMF oxidation, the TEMPO radicals immobilized on SBA-15 must be
dismutated to its oxoammonium form in the presence of acetic acid (AcOH) [20] (Fig. S4).
Previous work identified the dual role of BAIB as TEMPO regenerant and as AcOH source (via
ligand exchange), which was needed to ‘jump-start’ the oxidation reaction by formation of
oxoammonium. However, participation of BAIB in ligand exchange reaction with 5-HMF to
form AcOH was only minor, prompting the necessity of external AcOH addition [27].Results
(Fig. 7 and Table S2) confirmed that addition of small amount of AcOH (0.05 mmol) remarkably
increased the 2,5-DFF yields of both homogeneous (78%) and heterogeneous TEMPO-catalyzed
systems (73%).
<<Insert Figure 7>>
The small difference in 2,5-DFF yields between the homogeneous and heterogeneous
systems further affirms the benefit of using SBA-15 as the support material for TEMPO. The
catalyst support SBA-15 exerted minimum effect on theTEMPO due to uniform distribution of
active sites in its mesoporous structure. Moreover, the suitableorganosilane linker (CPTES) aids
to TEMPO moiety with its propyl chain by providing enough spaceto react by lessening any
steric hindrance in the vicinity. Thus, TEMPO-SBA-15closely mimicked the behavior of the
homogeneous catalyst system [45] in terms of 2,5-DFF yield. A similar heterogeneous catalyst
TEMPO-Silica using silica (fumed) (TEMPO loading 0.1 mmol/g) was compared for 5-HMF
oxidation resulting into 42 % 2,5-DFF yield (Fig. 7) with complete 5-HMF conversion (Table
S2). The possible reason could be the lesser exposure to active sites due to aggregation of silica
particles and the decreased diffusive transport of reaction components unlike in the TEMPO-
SBA-15.
19

384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
3.7.Stability and Recycling of TEMPO-SBA-15
The stability of TEMPO-SBA-15 was examined by repeated use of the catalyst in the same
reaction conditions. Recovered TEMPO-SBA-15 had similar catalytic performance with that of a
fresh catalyst even after several recycling runs (Fig. 8e). The 2,5-DFF yields were maintained
above 71% with minimal variation between each cycle. After five consecutive cycles, TEMPO-
SBA-15 did not reveal anynoteworthy form of chemical and structural deterioration as evidenced
by FT-IR and SEM analyses, respectively (Fig. 8a-8d).Recycling tests confirm the potential of
TEMPO-SBA-15for long term repeated use as a catalyst for5-HMF oxidation to 2,5-DFF.
<Insert Figure 8>>
4. Conclusion
The TEMPOcatalyst with organosilane linkerwas successfully immobilizedon SBA-15
support via silanization as confirmed by various characterization techniques. This metal-free
heterogeneous catalyst presented promising results by providing high 2,5-DFF yield in short
reaction time at ambient reaction conditions as compared to previously reportedreactionsystems.
The mesoporous SBA-15proved suitable as support of TEMPO; there was minimal reduction in
2,5-DFF yields and low propensity of the products and substrates forunwanted product
formation.Thus, heterogeneous TEMPO-SBA-15catalystprovides an effective route for 2,5-
DFFproduction, with reliable catalytic activity for repeated and long-term use.
5. Acknowledgements
20

407
408
409
410
This work was supported by the National Research Foundation of Korea(NRF) grant
funded by the Ministry of Science, ICT & Future Planning(No. 2012R1A2A1A01009683) and
the Ministry of Education(No. 2009-0093816).
21

411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
References
[1]
K.T. Hopkins, W.D. Wilson, B.C. Bender, D.R. McCurdy, J.E. Hall, R.R. Tidwell,
A. Kumar, M. Bajic, D.W. Boykin, J. Med. Chem. 41 (1998) 3872-3878.
M.D. Poeta, W.A. Schell, C.C. Dykstra, S. Jones, R.R. Tidwell, A. Czarny, M. Bajic,
M.Bajic, A. Kumar, D. Boykin, J.R. Perfect, Antimicrob. Agents Chemother. 42
(1998) 2495-2502.
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
D.T. Richter, T.D. Lash, Tetrahedron Lett. 40 (1999) 6735-6738.
A. Gandini, M.N. Belgacem, Prog. Polym. Sci. 22 (1997) 1203-1379.
Y. Wang, B. Liu, K. Huang, Z. Zhang, Ind. Eng. Chem. Res. 53 (2014) 1313-1319.
D. Ray, N. Mittal, W.J. Chung, Carbohydr. Res. 346 (2011) 2145-2148.
N. Mittal, G.M. Nisola, W.J. Chung, Tetrahedron Lett. 53 (2012) 31493155.
J.W. Reijendam, G.J. Heeres, M.J. Janssen, Tetrahedron Lett. 26 (1970) 1291-1301.
L. Cottier, G. Descotes, J. Lewkowski, R. Skowronski, Pol. J. Chem. 68 (1994) 693-
698.
[10]
L. Cottier, G. Descotes, J. Lewkowski, R. Skowronski, Org. Prep. Proced. Int. 27
(1995) 564-566.
[11]
[12]
[13]
[14]
W. Partenheimer, V.V. Grushin, Adv. Synth. Catal. 343 (2001) 102-111.
G.A. Halliday, R.J. Young, V.V. Grushin, Org. Lett. 5 (2003) 2003-2005.
J.P. Ma, Z.T. Du, J. Xu, Q.H. Chu, Y. Pang, ChemSusChem 4 (2011) 51-54.
C. Carlini, P. Patrono, A.M.R. Galletti, G. Sbrana, V. Zima, Appl. Catal. A: Gen.289
(2005) 197-204.
[15]
O.C. Navarro, A.C. Canos, S.I. Chornet, Top. Catal. 52 (2009) 304-314.
22

433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
[16]
A. Takagaki, M. Takahashi, S. Nishimura, K. Ebitani, ACS Catalysis 1 (2011) 1562-
1565.
[17]
[18]
Z.Z. Yang, J. Deng, T. Pan, Q.X. Guo, Y. Fu, Green Chem. 14 (2012) 2986-2989.
I. Sadaba, Y.Y. Gorbanev, S. Kegnaes, S.S.R. Putluru, R.W. Berg, A. Riisager,
ChemCatChem 5 (2013) 284-293.
[19]
[20]
G.D. Yadav, R.V. Sharma, Appl. Catal. B: Environ. 147 (2014) 293–301.
A.D. Mico, R. Margarita, L. Parlanti, A. Vescovi, G. Piancatelli, J. Org. Chem. 62
(1997) 6974-6977.
[21]
R.A. Sheldon, I.W.C.E. Arends, J. Mol. Catal. A: Chem. 251 (2006) 200-214.
J.M. Hoover, B.L. Ryland, S.S. Stahl, J. Am. Chem. Soc. 135 (2013) 2357-2367.
P.L. Bragd, A.C. Besemer, H. van Bekkum, J. Mol. Catal. A: Chem. 170 (2001) 35-
[22]
[23]
42.
[24]
[25]
A.E.J. de Nooy, A.C. Besemer, H. van Bekkum, Synthesis 10 (1996) 1153-1176.
L. Cottier, G. Descotes, J. Lewkowski, R. Skowronski, E. Viollet, J. Heterocycl.
Chem. 32 (1995) 927-930.
[26]
[27]
T.S. Hansen, I. Sadaba, E.J.G. Suarez, A. Riisager, Appl. Catal. A: Gen. 456 (2013)
44-50.
N. Mittal, G.M. Nisola, L.B. Malihan, J.G. Seo, S.P. Lee, W.J. Chung, Korean J.
Chem. Eng. 31 (2014) 1362-1367.
[28]
[29]
[30]
[31]
N. Tsubokawa, T. Kimoto, T. Endo, J. Mol. Catal. A: Chem. 101 (1995) 45-50.
Y. Wang, X. Song, S. Shao, H. Zhong, F. Lin, RSC Advances 2 (2012) 7693-7698.
B. Karimi, E. Badreh, Org. Biomol. Chem. 9 (2011) 4194-4198.
B. Karimi, H.M. Mirzaei, E. Farhangi, ChemCatChem 6 (2014) 758-762.
23

456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
[32]
[33]
[34]
D.P. Serrano, J. Aguado, R.A. Garcia, C. Vargas, Stud. Surf. Sci. Catal. 158 (2005)
1493-1500.
D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120
(1998) 6024-6036.
X. Wang, Y.H. Tseng, J.C.C. Chan, S. Cheng, Microporous Mesoporous Mater. 85
(2005) 241-251.
[35]
[36]
S.K. Kundu, J. Mondal, A. Bhaumik, Dalton Trans. 42 (2013) 10515-10524.
L. Zhang, C. Yu, W. Zhao, Z. Hua, H. Chen, L. Li, J. Shi, J. Non-Cryst. Solids 353
(2007) 4055-4061.
[37]
D.P. Quintanilla, I. Hierro, M. Fajardo, I. Sierra, J. Environ. Monit. 8 (2006) 214-
222.
[38]
[39]
[40]
A.G.S. Prado, J.A.A. Sales, C. Airoldi, J. Therm. Anal. Calorim. 70 (2002) 191-197.
Z. Zheng, J. Wang, M. Zhang, L. Xu, J. Ji, ChemCatChem 5 (2013) 307-312.
L.Y. Yuan, Y.L. Liu, W.Q. Shi, Z.J. Li, J.H. Lan, Y.X. Feng, Y.L. Zhao, Y.L.
Yuan, Z.F. Chai, J. Mater. Chem. 22 (2012) 17019-17026.
C. Pereira, K. Biernacki, S.L.H. Rebelo, A.L. Magalhaes, A.P. Carvalho, J. Pires, C.
Freire, J. Mol. Catal. A: Chem. 312 (2009) 53-64.
[41]
[42]
[43]
X. Wang, X. Ma, L. Sun, C. Song, Green Chem. 9 (2007) 695-702.
A. Dijksman, I.W.C.E. Arends, R.A. Sheldon, Org. Biomol. Chem. 1 (2003) 3232-
3237.
[44]
[45]
S.R. Kirumakki, N. Nagaraju, S. Narayanan, Appl. Catal. A: Gen. 273 (2004) 1-9.
B. Thierry, J.C. Plaquevent, D. Cahard, Tetrahedron: Asymmetry 14 (2003) 1671-
1677.
24

480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
Figure captions
Fig. 1 (a) Small angle powder XRD patterns and (b) N2 adsorption-desorption isotherms and
pore size distribution curves (inset) of SBA-15 and TEMPO-SBA-15.
Fig. 2 Thermogravimetric curves of SBA-15 and TEMPO-SBA-15.
Fig. 3 FT-IR spectra of (a) PTES-TEMPO , (b) SBA-15, and (c) TEMPO-SBA-15.
Fig. 4 SEM and FE-SEM images of (a), (c) SBA-15 and (b), (d) TEMPO-SBA-15 at different
magnifications.
Fig. 5 Comparison of homogeneous and heterogeneous TEMPO catalysts under oxygen and no-
oxygen atmosphere. Reaction conditions: 5-HMF (0.5 mmol), BAIB (0.75 mmol),ethyl acetate
(8 mL), RT, 1 h.
Fig. 6 Reaction profile of 5-HMF oxidation using TEMPO-SBA-15 as catalyst at different
temperatures. Reaction conditions: 5-HMF (0.5 mmol), TEMPO-SBA-15 (0.075 mmol
TEMPO), BAIB (0.75 mmol),ethyl acetate (8 mL), O2.
Fig. 7 Comparison of different reaction systems for the oxidation of 5-HMF using
homogeneous and heterogeneous TEMPO catalyst. Reaction conditions: 5-HMF (0.5 mmol),
25

502
503
504
505
506
507
508
509
510
511
TEMPO (0.075 mmol), BAIB (0.75 mmol, if used),AcOH (0.05 mmol, if used), ethyl acetate (8
mL), O2(if used), 1 h.
Fig. 8 Recycling results of TEMPO-SBA-15 catalyst. (a) SEM image and (b) FT-IR spectrum
of fresh catalyst, (c) SEM image and (d) FT-IR spectrum of used catalyst, and (e) 2,5-DFF yields
in 5-HMF oxidation over five recycling runs. Reaction conditions: 5-HMF (0.5 mmol), TEMPO-
SBA-15 (0.075 mmol TEMPO), BAIB (0.75 mmol),AcOH (0.05 mmol), ethyl acetate (8 mL),
O2, 40 oC, 1 h.
Scheme 1Synthesis of TEMPO-SBA-15 catalyst.
512
Scheme 2Oxidation of 5-HMF to 2,5-DFF over TEMPO-SBA-15 catalyst.
513
514
Table 1
Textural properties and TEMPO content of the SBA-15 and TEMPO-SBA-15 catalyst
Surface area
(m2/g)a
Pore volume
(cm3/g)b
TEMPO content
(mmol/g)d
Sample
SBA-15
Pore size (nm)c
883
526
1.24
0.72
5.6
5.5
-
TEMPO-SBA-15
0.3
515
516
517
518
a Calculated by the BET equation
b Single point pore volume calculated at a relative pressure P/Po of 0.99
c Adsorption average pore diameter
d Determined by elemental analysis, C/N ratio = 15.7
26

519
520
27

521
28

522
29