Copper-catalyzed oxidative methyl-esterification of 5-hydroxymethylfurfural using TBHP as an oxidizing and methylating reagent: A new approach for the synthesis of furan-2,5-dimethylcarboxylate

Article abstract of DOI:10.1016/j.jcat.2020.05.032

Catalytic conversion of 5-hydroxymethylfurfural (HMF) into furan-2,5-dimethylcarboxylate (FDMC) is of great significance in the production of polyethylene furanoate (PEF), a renewable biomass-derived polymer that can replace the fossil dependent polyethylene terephthalate (PET). Herein, for the first time, we report the synthesis of FDMC from oxidative methyl-esterification of HMF using tert-butyl hydroperoxide (TBHP) as an oxidizing and methylating reagent catalyzed by mesoporous alumina nanospheres-embedded with CuO nanoparticles (CuO/m-Al₂O₃). The CuO/m-Al₂O₃ catalysts with different copper contents were prepared by evaporation-induced self-assembly of a structure-directing agent (Pluronic P-123). The decomposition of P-123 during calcination in air results into the formation of a mesoporous structure with highly dispersed CuO nanoparticles. The as-prepared 6-CuO/m-Al₂O₃ exhibits excellent catalytic activity towards oxidative methyl-esterification of HMF into FDMC with 92% yield and turnover frequency (TOF) of 0.56 h^?1. Furthermore, oxidative methyl-esterification of a range of substrates through SP³ C[sbnd]H bond functionalization has also been demonstrated using the same catalyst.

Full text of DOI:10.1016/j.jcat.2020.05.032

Journal Pre-proofs
Copper-catalyzed oxidative methyl-esterification of 5-hydroxymethylfurfural
using TBHP as oxidizing and methylating reagent: A new approach for the
synthesis of furan-2,5-dimethylcarboxylate
Shyam Sunder R. Gupta, Ajayan Vinu, Mannepalli Lakshmi Kantam
PII:
S0021-9517(20)30213-X
https://doi.org/10.1016/j.jcat.2020.05.032
YJCAT 13762
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
27 January 2020
18 May 2020
21 May 2020
Please cite this article as: S. Sunder R. Gupta, A. Vinu, M. Lakshmi Kantam, Copper-catalyzed oxidative methyl-
esterification of 5-hydroxymethylfurfural using TBHP as oxidizing and methylating reagent: A new approach for
the synthesis of furan-2,5-dimethylcarboxylate, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.
2020.05.032
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Copper-catalyzed oxidative methyl-esterification of 5-
hydroxymethylfurfural using TBHP as oxidizing and methylating reagent:
A new approach for the synthesis of furan-2,5-dimethylcarboxylate
Shyam Sunder R. Gupta†, Ajayan Vinu‡ and Mannepalli Lakshmi Kantam†,*
† Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E),
Mumbai - 400019, Maharashtra, India.
‡ Global Innovative Center for Advanced Nanomaterials (GICAN), School of Engineering,
Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan,
NSW 2308, Australia.
Corresponding author E-mail:lk.mannepalli@ictmumbai.edu.in
Abstract: Catalytic conversion of 5-hydroxymethylfurfural (HMF) into furan-2,5-
dimethylcarboxylate (FDMC) is of great significance in the production of polyethylene
furanoate (PEF), a renewable biomass-derived polymer that can replace the fossil dependent
polyethylene terephthalate (PET). Herein, for the first time, we report the synthesis of FDMC
from oxidative methyl-esterification of HMF using tert-butyl hydroperoxide (TBHP) as an
oxidizing and methylating reagent catalyzed by mesoporous alumina nanospheres-embedded
with CuO nanoparticles (CuO/m-Al2O3). The CuO/m-Al2O3 catalysts with different copper
contents were prepared by evaporation-induced self-assembly of a structure-directing agent
(Pluronic P-123). The decomposition of P-123 during calcination in air results into the
formation of a mesoporous structure with highly dispersed CuO nanoparticles. The as-
prepared 6-CuO/m-Al2O3 exhibits excellent catalytic activity towards oxidative methyl-
esterification of HMF into FDMC with 92% yield and turnover frequency (TOF) of 0.56 h-1.
Furthermore, oxidative methyl-esterification of a range of substrates through SP3 C-H bond
functionalization has also been demonstrated using the same catalyst.
Keywords: HMF; FDMC; Biomass; Copper-catalyzed; oxidative methyl-esterification
1. Introduction
Sustainable production of fuels and chemicals from renewable biomass is highly demanding
to combat the rapid depletion of fossil resources and climate change due to carbon footprint.
Therefore catalytic valorization of biomass has become important and drawn considerable
attention of researchers worldwide.[1–6] Among the various biomass-derived molecules, 5-
hydroxymethylfurfural (HMF) is of special interest. HMF can be obtained from the acid-

catalyzed dehydration of lignocellulosic biomass and used for the production of 2,5-
furandicarboxylic acid (FDCA) through oxidation of its side-groups.[7–10] FDCA has been
recognized as a promising replacement for petroleum-based terephthalic acid in polymer
production, especially polyethylene furanoate (PEF).[11,12] PEF exhibits tensile strength,
glass transition temperature, enzymatic hydrolysis and gas-impermeable properties superior
to those of polyethylene terephthalate (PET).[13–15] Despite the significant potential of
FDCA, there have been a lot of difficulties in its direct use for PEF production because of its
quantitative isolation in high purity for polymerization, high boiling point and low solubility
in various industrial solvents.[16–18] Esterification of FDCA into furan-2,5-
dimethylcarboxylate (FDMC) is the most promising approach to overcome the limitations
associated with FDCA. FDMC possesses a low boiling point and a high solubility in most
solvents and can easily be purified by low-temperature sublimation method. In addition it can
be directly employed in the transesterification step with ethylene glycol to produce
PEF.[11,17] Therefore, one-pot synthesis of FDMC from HMF is a cutting-edge process that
is in high demand, since such process will enhance the efficiency and reduce the cost in PFE
production (Figure 1).
Fig. 1. One-pot synthesis of FDMC from HMF.
Considering these points, several research groups have been actively working on the
development of a facile approach for the synthesis of FDMC from HMF. For example, the
pioneering work on direct synthesis of FDMC from HMF using molecular oxygen as the
oxidant and sodium methoxide as a base additive in the presence of an Au/TiO2 catalyst was
reported by Christensen and co-workers.[19] Corma and co-workers developed gold
supported nano ceria catalyst for oxidative esterification of HMF using molecular oxygen and
eliminated the use of any base additive as Lewis acidity of the unsaturated cerium assisted in

alcohol oxidation.[20] Subsequently, various gold-based catalysts were developed for the
efficient production of FDMC from HMF.[21–26] Considering the high cost of gold, Deng et.
al. applied pyrolyzed CoxOy-N@C along with an equal amount of K-OMS-2 as the catalytic
system for oxidative esterification of HMF in the presence of K2CO3 using 10 bar molecular
oxygen and obtained the desired product FDMC in 96% yield.[27] Sun et. al. studied the
effect of pyridinic nitrogen content in CoOx-N/C towards its catalytic activity in oxidative
esterification of HMF along with α-MnO2 as a co-catalyst and concluded that pyridinic
nitrogen facilitates the dehydrogenation of the hydroxyl group in HMF thereby eliminating
the need of a base.[28] Recently, Sun et. al. prepared a MOF derived hollow yolk-shell
Co@CN material using SiO2 as a hard template and studied its catalytic activity in oxidative
esterification of HMF. It was reported that, the SiO2 shell prevents the aggregation of cobalt
nanoparticles during pyrolysis at high temperature and its removal by NaOH etching afforded
highly dispersed cobalt with a hollow yolk-shell structure.[29] Inspired by these results,
several other cobalt-based catalysts have been developed for oxidative esterification of HMF
affording FDMC in excellent yields.[30–33] The aforementioned processes for oxidative
esterification of HMF into FDMC involve the use of molecular oxygen as a cheap and easily
available oxidant. However, the requirement of high oxygen pressure is a critical drawback
from the practical point of view, as it requires the specific equipments and extreme safety
precautions.[34]
Herein, for the first time, we report on the synthesis of FDMC through oxidative methyl-
esterification of HMF using tert-butyl hydroperoxide (TBHP) as both oxidizing as well as
methylating agent catalyzed by mesoporous alumina nanospheres-embedded with CuO
nanoparticles. A series of CuO/m-Al2O3 with varying amount of CuO was prepared by
evaporation-induced self-assembly (EISA) of an structure-directing agent (P-123) in an
ethanolic solution containing aluminium isopropoxide, Cu(NO3)2.3H2O and nitric acid
(Figure 2). Calcination of the resultant gel in air atmosphere resulted into the decomposition
of P-123 affording formation of alumina nanospheres with highly ordered, hexagonally
arranged mesopores embedded with CuO nanoparticles.[35–38] In comparison to the
incipient wetness impregnation of conventional alumina with metal precursors, which often
leads to the lowered surface area due to pore blockage, the one-pot EISA method is suitable
to achieve high-quality mesopores with homogeneous dispersion of active metals.[38,39]
Furthermore, the embedding of active metals in alumina framework mitigates their
aggregation during the reaction and improves the catalyst recycling. Recently, Ghosh et. al.

reported the oxidative methyl-esterification of various derivatives of benzyl alcohol,
benzaldehyde and benzoic acid catalyzed by Pd/Cu2Cl(OH)3 using TBHP and K2CO3 as a
base additive.[40] Here, we report the base free as well as a noble metal-free catalytic system
for oxidative methyl-esterification of various derivatives of benzyl amine and toluene in
addition to the substrates reported by Ghosh et. al. Furthermore, we have also demonstrated
the one-pot oxidative methyl-esterification of a range of substrates involving SP3 C-H bond
functionalization for the first time establishing the wide applicability of the present synthetic
strategy. TBHP has been used as a multipurpose reagent in various organic transformations
[40–46]. Here also it plays a crucial role, it acts as an oxidant as well as a methyl source in
oxidative methyl-esterification of HMF and eliminates the need of a stoichiometric amount of
base additive as the corresponding product does not lower the pH of the reaction medium.
Fig. 2. Graphical representation of CuO/m-Al2O3 catalyst preparation.
2. Experimental details
2.1 Catalyst preparation
A series of CuO/m-Al2O3 catalyst with different copper contents (2-CuO/m-Al2O3, 4-CuO/m-
Al2O3, 6-CuO/m-Al2O3 and 8-CuO/m-Al2O3 containing 2, 4, 6 and 8 wt% of copper oxide
respectively) have been prepared by evaporation-induced self-assembly method in
conformity with a previous study with slight modifications [35]. In a general synthesis of 6-
CuO/m-Al2O3, Pluronic P123 (4 g) was transferred into a beaker containing 40 mL of ethanol

and stirred at room temperature until it gets dissolved completely. Then, 65% nitric acid (7
mL) was added to another beaker containing aluminium isopropoxide (8 g) and ethanol (40
mL) and stirred until complete dissolution of aluminium isopropoxide. Addition of nitric acid
into the mixture of aluminium isopropoxide and ethanol was exothermic hence; the beaker
was placed in a cold water bath. The two solutions were mixed with continuous stirring and
Cu(NO3)2.3H2O (0.4 g) was added to the resulting mixture. The obtained mixture was stirred
o
at room temperature for 48 h and then placed in an oven maintained at 60 C for ethanol
evaporation for 72 h. A blue coloured gel was obtained after ethanol evaporation which was
calcined in air atmosphere using a tubular furnace at 400 oC for 4 h with a heating rate of 1 oC
per min. Similarly, the other catalysts were also prepared by varying the quantity of cupric
nitrate.
2.2 Procedure for oxidative methyl-esterification reaction
The liquid phase oxidative methyl-esterification of various substrates was carried out in a 15
mL glass vial equipped with a magnetic stirrer. In a general procedure, 1 mmol of the
substrate was added to the reaction vial containing 3 mL of dimethyl sulfoxide (DMSO), 1
mL of distilled water and 10 mg of the catalyst. To the resulting mixture, 70% aqueous TBHP
(4 mmol - 12 mmol) was added dropwise under continuous stirring. The addition of TBHP
was exothermic. The reaction vial was sealed with a Teflon septum fitted aluminium cap and
immersed in an oil bath maintained at 100 oC. The reaction was continued for 20 h. After the
reaction, the catalyst was isolated by centrifugation and the filtrate was mixed with 100 mL
of water in a 250 mL separating funnel. The organic compounds were extracted from the
aqueous mixture using ethyl acetate (50 mL x 2 times). The organic portion was combined
and dried over Na2SO4 and evaporated under vacuum using a rotator evaporator to obtain the
crude product. The crude products were purified by column chromatography using silica gel
(100-200 mesh) and a mixture of petroleum ether and ethyl acetate. The products were
initially identified by GC-MS and finally confirmed by NMR spectroscopy.
3. Results and discussion
3.1 Catalyst characterization
Phase identification of the as-prepared catalysts was done by recording their powder XRD
patterns and represented in Figure 3. The sharp diffraction peaks at 35.4o and 38.6o confirm
the presence of highly crystalline CuO nanoparticles (ICDD PDF No. 80-1268). The intensity
of the diffraction peaks corresponding to CuO in 8-CuO/m-Al2O3, 6-CuO/m-Al2O3 and 4-

CuO/m-Al2O3 reduced gradually due to the decreased copper content (Figure 3). The CuO
characteristic diffraction peaks were absent in 2-CuO/m-Al2O3 catalyst. This could be
possibly due to the small particle size of CuO or high dispersion of CuO inside the mesopores
of the alumina making it unavailable for X-ray diffraction. The absences of characteristic
diffraction peaks of Al2O3 suggest the formation of amorphous alumina [37]. Calcination at
o
400 C afforded the decomposition of template P123 resulting into the formation of highly
ordered hexagonal mesopores in alumina which was confirmed by recording the small-angle
X-ray scattering (SAXS) pattern of 6-CuO/m-Al2O3 (Figure S1, ESI). The sample 6-CuO/m-
Al2O3 shows a sharp X-ray scattering peak at 0.2o confirming the formation of mesoporous
structure [37]. The combination of powder XRD and SAXS results confirm the formation of
mesoporous alumina with crystalline CuO nanoparticles on its surface.
Fig. 3. XRD patterns of the as-prepared catalysts.
Elemental composition and chemical state of the elements in 6-CuO/m-Al2O3 was studied by
XPS analysis (Figure 4). The survey spectrum (Figure 4a) shows the presence of Cu, Al and
O elements in 6-CuO/m-Al2O3. The two peaks at 934.7 eV and 954.9 eV in Cu 2p high-
resolution scan (Figure 4b) corresponding to Cu 2p3/2 and Cu 2p1/2 suggest the presence of
Cu2+. The strong shake-up satellite peaks at 943.9 eV, and 962.5 eV confirms the oxidation
state of copper as Cu2+ [47]. The bivalent oxidation state of copper was further confirmed by
Cu LMM spectrum (Figure S10, ESI). The Cu LMM spectrum shows a peak at 569 eV,

which corresponds to CuO species [48]. The two peaks in high-resolution Al 2p spectra
(Figure 4d) indicate the presence of two different forms of aluminium in 6-CuO/m-Al2O3.
The low binding energy peak at 74.2 eV corresponds to Al-O (Al2O3), whereas the higher
binding energy peak at 77.3 eV can be assigned to Al-O (CuAl2O4) [49].
Fig. 4. XPS of 6-CuO/m-Al2O3 catalyst (a) survey scan, (b) Cu 2p, (c) O 1s and (d) Al 2p
high-resolution scans.
Surface morphology of the as-prepared 6-CuO/m-Al2O3 was examined by SEM and TEM
analysis. FEG-SEM images (Figure 5 a&b) reveal the formation of spherical shaped alumina
with particles having sizes in the range of 100 to 200 nm which is in good agreement with the
earlier report [50]. The formation of spherical shaped alumina can be attributed to the fast
hydrolysis of alumina precursor, which gets attached to the outer part of the spherical micelle
[51]. The low to high magnification TEM images (Figure 5 c&d) admit the formation of CuO
nanoparticles on the surface of alumina nanospheres. The presence of CuO particles on
alumina nanospheres was further confirmed by HRTEM analysis. The interplanar spacing

was found to be around 0.25 nm (Figure 5e) which is characteristic of the CuO (111) plane.
The concentric rings in selected area electron diffraction (SAED) pattern of 6-CuO/m-Al2O3
(Figure 5f) suggest the polycrystalline nature of the CuO nanoparticles. Elemental mapping
of 6-CuO/m-Al2O3 using TEM (Figure 6) shows the uniform dispersion of CuO throughout
the alumina support. The elemental analysis (Figure S2, ESI) and EDS line scan profile
(Figure S3, ESI) confirms the formation of 6-CuO/m-Al2O3 with a high purity and uniform
distribution of elements throughout the sample.
Fig. 5. FEG-SEM images (a) and (b), TEM images (c) and (d), HRTEM image (e) and SAED
pattern (f) of 6-CuO/m-Al2O3 catalyst.

Fig. 6. TEM image (a) and corresponding Cu (b), Al (c) and O (d) elemental mappings in 6-
CuO/m-Al2O3 catalyst.
The nitrogen adsorption-desorption isotherm (Figure S4a, ESI) with hysteresis loop admits
the presence of mesoporous structures in 6-CuO/m-Al2O3 resulting into a large BET surface
area of 340 m2/g with a total pore volume of 0.76 cm3/g. The pore size distribution curve
shows that the 6-CuO/m-Al2O3 has an average pore diameter of 4 nm (Figure S4b, ESI),
which is in good agreement with the earlier report [38]. XRF spectra of 6-CuO/m-Al2O3
(Figure S5, ESI) display a high-intensity peak at 1.5 KeV corresponding to Al-Kα and a low-
intensity peak at 8.04 KeV corresponding to Cu-Kα. The XRF quantification result reveals the
2, 4, 6 and 8 wt% loadings of CuO in 2-CuO/m-Al2O3, 4-CuO/m-Al2O3, 6-CuO/m-Al2O3 and
8-CuO/m-Al2O3 catalysts respectively (Figure S5, ESI). TGA analysis (Figure S6, ESI)
shows 15 wt% loss of the material at 100 oC which is possibly due to the removal of adsorbed
water molecules present in the pores of 6-CuO/m-Al2O3. Thus, the results obtained from
various characterization techniques suggest the successful formation of a mesoporous
structure with embedded CuO nanoparticles throughout the alumina nanospheres.
3.2 Catalytic activity
3.2.1 Oxidative methyl-esterification of HMF
Catalytic performance of a series of as-prepared CuO/m-Al2O3 catalyst containing different
amounts of CuO was evaluated towards oxidative methyl-esterification of HMF using TBHP,

and the results are presented in Table 1. The TBHP was used in excess during the whole
process of the reaction. Hence the reaction can be assumed to have first-order kinetics [52].
The normalized initial rates of FDMC formation were calculated to arrive at a meaning
comparison between the as-prepared catalysts [53,54]. The oxidative methyl-esterification of
HMF using 2-CuO/m-Al2O3 as a catalyst provided 50 mol L-1 h-1 molCuO-1 normalized rate of
FDMC formation with a TOF of 0.56 h-1 (Table 1, entry 1). The higher normalized rate of
FDMC formation and TOF was obtained when 4-CuO/m-Al2O3 was used as a catalyst (Table
1, entry 2). The 6-CuO/m-Al2O3 provided the highest normalized rate of FDMC formation
(200 mol L-1 h-1 molCuO-1) and TOF (0.56 h-1). The use of 8-CuO/m-Al2O3 as a catalyst
lowered normalized rate of FDMC formation (175 mol L-1 h-1 molCuO-1) as well as TOF (0.56
h-1). The lowered normalized rate and TOF with 8-CuO/m-Al2O3 catalyst could be possibly
due to the formation of larger CuO nanoparticles as a result of higher percentage loading of
metal on the surface of alumina nanospheres. Next, the catalytic activity is completely
because of the CuO nanoparticles was confirmed when m-Al2O3 without any copper loading
was used in the reaction (Table 1, entry 5). A blank reaction without using any catalyst
showed that no FDMC was formed even after 20 h which clearly indicates very slow rate of
the reaction in absence of catalyst (Table 1, entry 6). The more details for TON and TOF
calculations are provided in Table S1 (ESI).
Table 1. Catalytic performance of the as-prepared catalysts towards oxidative methyl-
esterification of HMF.a
Entry
Catalyst
CuO
FDMC formation
Normalized rate d
FDMC TOF
yield e (h-1)
(%)
loadingb
rate c
(mol L-1 h-1 molCuO
)
-1
(wt %) (mol L-1 h-1 gcat
)
-1
1
2
3
4
5
6
2-CuO/m-Al2O3
4-CuO/m-Al2O3
6-CuO/m-Al2O3
8-CuO/m-Al2O3
m-Al2O3
2
4
6
8
-
0.005
0.075
0.15
0.175
-
50
150
200
175
-
12
52
92
93
-
0.20
0.47
0.56
0.42
-
Blank
-
-
-
-
-
a
Reaction conditions: HMF, 1 mmol; catalyst, 0.1 g; TBHP, 20 mmol; solvent, DMSO:
c
Water 3:1 mL; temperature, 100 oC; time, 20 h; rpm, 800. b Estimated from XRF analysis.
Initial rates calculated as an average value between 1 and 2 h reaction time with a FDMC
yield of ~ 10%. d rates normalized to the molar concentration of CuO. e Determined from GC.

The effect of catalyst loading on oxidative methyl-esterification of HMF was
comprehensively studied by varying the quantity of 6-CuO/m-Al2O3 from 13 to 33 g L-1
(Figure 6a). Increasing the catalyst loading from 13 to 26 g L-1 increased the formation rate
and molar yield of FDMC due to the increased number of active sites in the reaction. Further
increasing the loading of 6-CuO/m-Al2O3 to 33 g L-1 further increases the initial rate of
formation of FDMC however, the molar yield of FDMC remained constant. This indicates
that the number of active sites was sufficient to obtain the maximum molar yield of FDMC.
Additionally, the initial rate of FDMC formation was plotted against the loading of 6-CuO/m-
Al2O3 (Figure 6b). The initial rate of FDMC formation was found to be directly proportional
to the loading of the catalyst due to the proportional increase in the number of active sites.
The linear relationship between 6-CuO/m-Al2O3 loading and rate of formation of FDMC
suggests the absence of mass transfer resistance. Next the effect of various solvents on
oxidative methyl-esterification of HMF was studied (Figure S9, ESI). The mixture of DMSO
and water in 3:1 ratio provided the maximum yield of the desired product. The oxidative
methyl-esterification of HMF was carried out at different agitation speeds ranging from 600
to 1000 rpm. The molar yield of FDMC was found to be constant at different agitation speeds
(Figure 7a). This indicates that the rate of external diffusion was higher as compared to the
rate of formation of FDMC. Therefore, all further reactions were carried out at an agitation
speed of 800 rpm.
Fig. 6. Effect of 6-CuO/m-Al2O3 loading on oxidative methyl-esterification of HMF.
Reaction conditions: HMF, 1 mmol; 6-CuO/m-Al2O3, 13 to 26 g L-1; TBHP, 20 mmol;
DMSO:H2O, 3:1mL; temperature, 100 oC; time, 20 h; rpm, 800. Initial rates calculated as an
average value between 1 and 2 h reaction time with an FDMC yield of ~ 10%.

The effect of reaction temperature on oxidative methyl-esterification of HMF was studied. As
the temperature increases from 353 to 383 K, the initial rate of FDMC formation also
increases (Figure 7c). This indicates that the reaction was intrinsically kinetically controlled.
Although the rate of reaction was found to increase with an increase in temperature, the molar
yield of FDMC was found to be almost constant at reaction temperatures of 373 and 383 K
(Figure 7b). Therefore, 373 K was considered as the optimum temperature for oxidative
methyl-esterification of HMF. The Arrhenius plot was obtained to calculate the apparent
activation energy (Ea) by plotting ln (initial rates) versus reciprocal of temperatures in the
range of 353 to 383 K (Figure 7d) [55]. The apparent activation energy (Ea) was calculated
from the slop of the plot was found to be 6.8 kcal/mol.
Fig. 7. Effect of reaction temperature on oxidative methyl-esterification of HMF.
Reaction conditions: HMF, 1 mmol; 6-CuO/m-Al2O3, 26 g L-1; TBHP, 20 mmol;
DMSO:H2O, 3:1mL; temperature, 353 - 383 K; time, 20 h; rpm, 800. Initial rates calculated
as an average value between 1 and 2 h reaction time with a FDMC yield of ~ 10%.
3.2.2 Oxidative methyl-esterification of various biomass-derived molecules

To establish the high efficiency and general applicability of the present synthetic
methodology in biomass valorization, various furanic compounds derived from biomass were
successfully converted into the corresponding methyl-esters using optimized reaction
conditions and presented in Table 2. A higher yield of FDMC (96%) was obtained when 2,5-
furandicarboxylic acid was used as the starting material (Table 2, entry 3). Oxidative methyl-
esterification of 2,5-dimethylfuran and 2-methylfuran involving SP3 C-H bond
functionalization also afforded the good yield of the corresponding products (Table 2, entry 4
and 5). The optimized quantity of TBHP for oxidative methyl-esterification of HMF, 2,5-
DFF, FDCA, 2,5-DMF, 2-MF, 2-furoic acid, furfuryl alcohol and furfural is 8 mmol, 8 mmol,
6 mmol, 12 mmol, 6 mmol, 4 mmol, 4 mmol and 4 mmol, respectively.

Table 2. Oxidative methyl-esterification of various biomass-derived furanic compounds.a
a
Reaction conditions: Substrate, 1 mmol; 6-CuO/m-Al2O3, 8 mol%; TBHP, 4-12 mmol;
DMSO:H2O, 3:1 mL; temperature, 100 oC; time, 20 h. b GC-MS yields.
3.2.3 Oxidative methyl-esterification of various homocyclic aromatic compounds
To further extend the substrate scope, we explored the oxidative methyl-esterification of
various homocyclic aromatic compounds using the present synthetic system. As shown in
Scheme 1, various derivatives of benzaldehyde, benzyl alcohol, benzylamine, benzoic acid
and toluene successfully undergo oxidative methyl-esterification reaction in the presence of 4
mmol, 6 mmol, 6 mmol, 4 mmol and 6 mmol of TBHP respectively providing good to
excellent yields of the corresponding products.

Scheme 1. Oxidative methyl-esterification of various substrates.a
a
Reaction conditions: Substrate, 1 mmol; 6-CuO/m-Al2O3, 8 mol%; TBHP, 4-6 mmol;
DMSO:H2O, 3:1 mL; temperature, 100 oC; time, 20 h; rpm, 800. b isolated yields.
3.3 Mechanistic investigation and tentative reaction mechanism
To understand the reaction mechanism involved in oxidative methyl-esterification of HMF,
some controlled experiments were carried out as presented in Scheme 2. The reaction
produced 84% of 1-methoxy-2,2,6,6-tetramethylpiperidine and only 8% of the desired
product FDMC when 1 mmol of radical (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO)
was introduced in the reaction under optimized conditions (Scheme 2b). This finding

confirms the formation of methyl radical in the reaction which on successively coupling with
TEMPO radical afforded the 1-methoxy-2,2,6,6-tetramethylpiperidine in major amount. Next
to figure out whether the methyl radical comes from the TBHP or DMSO, we carried out the
reaction using water as the only solvent instead of using DMSO and water mixture. An 87%
yield of FDMC was obtained when water alone was used as a solvent for the reaction,
confirming the generation of methyl radical from TBHP (Scheme 2c).
Scheme 2: Control experiments and the effect of radical inhibitor.
On the basis of the results obtained from the controlled experiments and earlier reports, a
tentative reaction mechanism is proposed in Scheme 3 [40,43]. Initially, the reaction of
TBHP and Cu(II) generates a tert-butoxyl radical and Cu(III) species in the mixture. The
Cu(III) species on further reaction with an another molecule of TBHP get reduced to Cu(II)
and simultaneously generates a tert-butylperoxyl radical (Step I, Scheme 3) [56]. In the next
step, tert-butoxyl radical gets converted into acetone and simultaneously generated a methyl
radical. The substrate, HMF or 2,5-dimethylfuran gets oxidized to FDCA in presence of the
oxidant TBHP. Subsequently, the reaction of 2,5-furandicarboxylic with tert-butoxyl or tert-
butylperoxyl radicals generates acyloxyl radical intermediate (Step II, Scheme 3). Finally, the
successive coupling of the acyloxyl radical with methyl radicals results in the formation of
the desired product FDMC (Step III, Scheme 3).

Scheme 3. The tentative reaction mechanism for the oxidative methyl-esterification of HMF.
3.4 Catalyst stability and reusability
The reusability of 6-CuO/m-Al2O3 in oxidative methyl-esterification of HMF was studied and
the results are shown in Figure 8. During the recycling study, the catalyst was separated from
the reaction mixture by centrifugation at 10000 rpm for 10 min and washed with water and
then with ethanol by centrifugation and redispersion. The isolated catalyst was then dried
o
overnight in a vacuum oven at 100 C and then used for the next catalytic run. The
conversion of HMF remained 100% during the recycling. However, the Yield of FDMC
decreased slightly from 92% to 85% similarly the TOF was also decreased from 0.56 h-1 to
0.51 h-1 after 4 consecutive runs (Figure 8). This slight decrease in yield, as well as TOF, can
be attributed to the slight loss of catalyst during recycling study. The stability of the catalyst
was examined by recording the XRD of 6-CuO/m-Al2O3 after recycling for 4 times. The
reused 6-CuO/m-Al2O3 shows the diffraction peaks similar to the fresh catalyst (Figure S7,

ESI). Furthermore, the SEM images of the recycled catalyst (Figure S8, ESI) shows no
change in the morphology of the catalyst. The XRD and SEM analysis suggest the high
stability of the 6-CuO/m-Al2O3 catalyst.
Fig. 8. Reusability of 6-CuO/m-Al2O3 catalyst in oxidative methyl-esterification of HMF.
4. Conclusions
We have demonstrated for the first time the one-pot synthesis of FDMC from oxidative
methyl-esterification of HMF using TBHP as an oxidizing and methylating agent in the
presence of a 6-CuO/m-Al2O3 catalyst. The mesoporous alumina-nanospheres embedded with
CuO nanoparticles were prepared from evaporation-induced self-assembly of a structure-
directing agent in ethanolic solution. The as-prepared catalyst exhibited excellent catalytic
activity in oxidative methyl-esterification of HMF, providing 92% yield of FDMC. The
apparent activation energy (Ea) for oxidative methyl-esterification of HMF was calculated
from the Arrhenius plot was found to be 6.8 kcal/mol. We have also demonstrated the
oxidative methyl-esterification of a range of substrates through SP3 C-H bond
functionalization using the same catalyst. The mechanistic investigation revealed the reaction
proceeds through the radical mechanism, and the methyl group in the product comes from
TBHP indicating its dual role in the reaction. Besides the biomass-derived molecules,
oxidative methyl-esterification of various derivatives of benzaldehyde, benzyl alcohol,
benzylamine, benzoic acid and toluene also provided good to the excellent yield of the
corresponding esters. Thus the wide applicability of the present protocol has been established
in oxidative methyl-esterification reaction.

Acknowledgements:
Author SSG is thankful to the Godrej Industries Ltd., India for providing the senior research
fellowship. Author MLK is greatly thankful to the Godrej Industries Ltd., India for Dr. B.P.
Godrej Professor of Green Chemistry and Sustainability Engineering. Author MLK also
acknowledge to the Science and Engineering Research Board, DST, India for J.C. Bose
national fellowship. Authors are greatly thankful to Prof. B.M. Bhanage for characterization
supports.
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Graphical Abstract:

Highlights:




Mesoporous alumina nanospheres-embedded with CuO nanoparticles.
A new approach for synthesis of FDMC from HMF using TBHP.
TBHP acts as an oxidizing and methylating reagent.
Oxidative methyl-esterification involving SP3 C-H bond functionalization.

Declaration of interest:
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.