Hetero-mixed TiO2-SnO2 interfaced nano-oxide catalyst with enhanced activity for selective oxidation of furfural to maleic acid

Article abstract of DOI:10.1016/j.inoche.2021.108637

Herein we report on the catalytic activity of hetero-mixed TiO₂-SnO₂ nano-oxide catalyst for the selective liquid-phase oxidation of furfural to maleic acid using H₂O₂ oxidant. The high surface area and strong interaction of the two oxides with modified electronic structure manifested enhanced effective oxygen vacancies, and redox activity performance of the TiO₂-SnO₂ catalyst for furfural oxidation reaction. The structure of the catalyst was investigated by the powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transition electron microscopy (HRTEM), electron paramagnetic resonance (EPR) and Brunauer-Emmett-Teller (BET) surface area analyser techniques. The interfaced TiO₂-SnO₂ oxide catalyst was more catalytically active than its single counterpart SnO₂ and TiO₂ oxides to give a furfural conversion of 96.2% at up to 63.8% yield of maleic acid. The catalytic performance shown by TiO₂-SnO₂ present encouraging prospects for an economical solid metal oxide catalyst to access biobased maleic acid from renewable biomass-derived furfural.

Full text of DOI:10.1016/j.inoche.2021.108637

Inorganic Chemistry Communications 129 (2021) 108637
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Hetero-mixed TiO -SnO interfaced nano-oxide catalyst with enhanced
2
2
activity for selective oxidation of furfural to maleic acid
a,
b
a,
b,
*
b
Petrus M. Malibo , Peter R. Makgwane
, Priscilla G.L. Baker
a
Centre for Nanostructures and Advanced Materials (CeNAM), Council for Scientific and Industrial Research (CSIR), Pretoria 0001, South Africa
Department of Chemistry, University of the Western Cape, Robert Sobukwe Drive, Private Bag X17, Bellville 7535, South Africa
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Herein we report on the catalytic activity of hetero-mixed TiO -SnO2 nano-oxide catalyst for the selective liquid-
2
Heterostructure oxide
Tin dioxide
phase oxidation of furfural to maleic acid using H
two oxides with modified electronic structure manifested enhanced effective oxygen vacancies, and redox ac-
tivity performance of the TiO -SnO catalyst for furfural oxidation reaction. The structure of the catalyst was
2 2
O oxidant. The high surface area and strong interaction of the
Titanium dioxide
Oxidation
2
2
investigated by the powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution
transition electron microscopy (HRTEM), electron paramagnetic resonance (EPR) and Brunauer-Emmett-Teller
Furfural
(
BET) surface area analyser techniques. The interfaced TiO
than its single counterpart SnO and TiO oxides to give a furfural conversion of 96.2% at up to 63.8% yield of
maleic acid. The catalytic performance shown by TiO -SnO present encouraging prospects for an economical
2 2
-SnO oxide catalyst was more catalytically active
2
2
2
2
solid metal oxide catalyst to access biobased maleic acid from renewable biomass-derived furfural.
1
. Introduction
titanium-silicate (TS-1) was evaluated for the oxidation of furfural with
H
2
O
2
to achieve maleic acid yield of 78% but the catalyst showed
leaching, which affected its recycling activity [11]. Other heterogeneous
solid catalysts have been studied for furfural oxidation with H and O
Selective oxidation of furfural to maleic acid and maleic anhydride is
one of the important biomass conversion chemical processes in the
production of renewable biobased chemicals [1]. Both maleic acid and
maleic anhydride can be converted to bio-succinic acid, a precursor to
2
O
2
2
to maleic acid but still show low yields of maleic acid and/or poor
recyclability [9,12,13]. Based on the previous reported catalytic activ-
1
,4-butanediol intermediate, which are both used in the synthesis of
ities of TiO
their interfaced oxides together with the use of H
access to economical synthesis route to biobased maleic acid from
furfural oxidation. The activity enhancement effect of TiO -SnO could
and SnO
2
and SnO
2
in oxidation reactions [11,14,15], the design of
numerous biodegradable bioplastic polymers [2,3]. Industrially, the
production of maleic acid is based on the maleic anhydride dehydration
reaction process, which is accessed from the gas-phase oxidation of
2 2
O
could provide an
2
2
butane or benzene substrates over preferred V
4,5].The process is fossil-based consisting of several energy intensive
steps masked by co-production of green house gases such as CO and CO,
2
O
5
oxide based catalysts
result from Lewis acidic sites associated with both TiO
2
2
,
including the Ti4 /Ti redox reactivity and oxygen vacancies [16–20].
+
3+
[
2
In this work, we demonstrate the enhanced catalytic activity of heter-
thus rendering the process economically costly and environmentally
unfriendly [5].
ostructure TiO
2
-SnO
2
interfaced catalyst in the oxidation of furfural to
maleic acid using H
2 2
O . To understand the basis of the catalytic effect,
Recently, there has been a growing interest in developing both liquid
and gas-phase oxidation conversion of furfural to biobased maleic acid
the structure characteristics of the catalysts was studied by X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron
paramagnetic resonance (EPR), high-resolution transition electron mi-
croscopy (HRTEM), and Brunner-Emmett-Teller (BET) nitrogen sorption
techniques.
and maleic anhydride, respectively using H
2
O
2
and molecular O
See Scheme 1). A bifunctional redox-acidic sites waste-derived poly-
styrene sulphonic acid) catalyst has been designed and evaluated in the
oxidant to achieve a maleic
acid yield of 34% [10]. The well-known epoxidation active catalyst,
2
[6–9]
(
(
2 2
furfural oxidation to maleic acid using H O
*
Corresponding authors.
E-mail addresses: pmakgwane@csir.co.za, makgwane.peter@gmail.com (P.R. Makgwane).
https://doi.org/10.1016/j.inoche.2021.108637
Received 12 February 2021; Received in revised form 20 April 2021; Accepted 25 April 2021
Available online 29 April 2021
1
387-7003/© 2021 Elsevier B.V. All rights reserved.

P.M. Malibo et al.
Inorganic Chemistry Communications 129 (2021) 108637
Fumaric acid
2-(5H)furanone
H O
H O
H O
2 2
2
2
2
2
Furfural
5-Hydroxy-2(5H)-furanone
Maleic acid
Oxalic acid
Scheme 1. Plausible reaction pathways for oxidation of furfural to intermediates, maleic acid and other products.
Succinic acid
2-Furoic acid
2 2
Fig. 1. Characterisation of TiO -SnO catalyst. (a) XRD patterns; XPS spectra of (b) Sn 3d peaks, (c) Ti 2p peaks, (d) O 1s peaks; and (e) EPR profiles.
2
. Experimental methods
2 2 2
with respect to single TiO and SnO oxides as shown in Fig. 1a. SnO is
characterised by a tetragonal rutile structure with main peaks at 2θ of
◦
◦
◦
◦
The synthesis of the heterostructured TiO
2
-SnO
2
and single SnO
2
and
26.9 , 34.2 and 51.8 together with additional peaks at 2θ of 29.3 and
◦
TiO nano-oxide catalysts was carried by a solvothermal microwave-
2
36.3 , which are due to SnO [21]. The XRD of TiO
2
shows formation of
2
peaks are not well
assisted heating method using solvent mixture of ethanol and ethylene
glycol. The obtained and dried solids were calcined in a muffle furnace
the anatase phase. For TiO
resolved due to overlap with the SnO
The XPS spectra of Sn 3d, Ti 2p and O 1s peaks of the heterostructure
TiO -SnO catalysts are illustrated in Fig. 1b-d. The Sn 3d spectrum of
SnO
(Fig. 1b) shows the spin–orbit doublet corresponding respectively
to 3d5/2 and 3d3/2 peaks of Sn4 at 486.6 eV and 495.0 eV [22]. The Ti
2p of TiO
(Fig. 1c) display a spin–orbit doublet of peaks due to 2p3/2
2
-SnO
2
catalyst, the TiO
2
peaks.
◦
◦
under air condition at a heating rate of 3 C/min from 30 to 500 C and
◦
held at 500 C for 4 h. The oxidation of furfural was carried out in a 25
2
2
ml flask fitted with a magnetic stirrer bar and a reflux condenser.
Furfural (5 mmol), catalyst (100 mg) and co-solvent mixture (10 ml) of
2
+
5
ml H
2 3
O and 5 ml acetonitrile (CH CN) were placed in the flask and
2
H
2
O
2
(25 mmol, 30% in H
2
O) was added to the mixture. The flask was
and 2p1/2 at 458.6 eV and 464.2 eV and a satellite peak at 471.8 eV
showing presence of Ti4 [23]. The O 1s peak of SnO
+
then quickly sealed with balloon and placed in a sand bath preheated to
2
at 530.4 eV
◦
2ꢀ
6
0
C under reflux conditions. This mixture was then stirred continu-
indicate the existence of O lattice oxygen atoms (Fig. S2) [24]. Both
◦
ously at 1000 rpm and kept at 60 C for 24 h. The separated liquid
oxidation product from the solid catalysts was analysed by high per-
formance liquid chromatography (HPLC). The elaborative catalysts
synthesis and characterisation methods, including detailed oxidation
products analysis are presented in the electronic supporting information
Sn 3d and O1s shows the peaks shifting respectively in TiO
lower and higher BE regions (Fig. 1a and d). The peaks shifting is an
indication of the strong interaction between TiO and SnO . The similar
shifting of the XPS peaks observed in the heterostructure of TiO -SnO
catalyst has been reported to arise from the electrostatic and electronic
effects [25–27]. In the case of TiO -SnO hetero-interfaced catalyst, the
2 2
-SnO to
2
2
2
2
(
ESI).
2
2
higher electronegativity of Ti could results in electrons being pulled
away from Sn toward Ti (i.e. strong Ti and Sn interaction referred to),
and thus causing the peaks shifting which are accompanied by the
modified electronic and chemical state.
3
. Results and discussion
Fig. 1 summarizes the XRD, XPS and EPR characterisation results of
the TiO
2
-SnO
2
based catalysts. The XRD analysis confirmed the crystal
-SnO interfaced catalyst
Fig. 1e shows the EPR spectra of TiO
2 2
-SnO and its corresponding
structure and phase compositions of the TiO
2
2
single oxides to investigate their interface effect on defects structure and
2

P.M. Malibo et al.
Inorganic Chemistry Communications 129 (2021) 108637
SnO2
SnO2
SnO2
TiO2
TiO
2
-SnO
2
TiO2
TiO -SnO2
2
TiO2
TiO -SnO2
2
1
0 1/nm
10 1/nm
10 1/nm
2 2
Fig. 2. TEM, SAED and HRTEM micrograph images of the TiO -SnO catalysts.
Table 1
2 2 2
N sorption and textural analysis of the TiO -SnO catalysts.
2
3
a
Catalysts
SnO
S
BET (m /g)
D
pore (nm)
V
pore (cm /g)
Crystallite size (nm)
Adsorp
Desorp
SnO2
SnO2
3.53 nm
2
13.2
7.6
9.5
0.031
0.017
0.20
6.1
6.9
5.9
TiO
TiO
2
9.0
2
-SnO
2
72.6
11.0
18.55 nm
a
determined by XRD using Scherrer equation. SBET = surface area; Dpore
=
pore diameter; Vpore = pore volume.
oxygen vacancies properties. Both Sn4 and Sn cations are EPR silent
+
2+
thus, the observed signals for SnO are due to the paramagnetic defect
2
Adsorp
Desorp
TiO2
TiO2
centres. These are confirmed by anisotropic resonance lines at g =
ꢀ
1
.971, g = 1.978 and g = 1.991, which are attributed to the V
O
para-
magnetic centres created by the single-electron trapped inside the oxy-
gen vacancies (V ) [22]. The EPR signals for TiO are due to the charge
separation, which result from illumination at low temperature. These
TiO EPR photo-induced signals are at g = 1.993 and g
= 1.951, g
.044, respectively. The g and g are due to the electron trapping sites
in the anatase/rutile phase of TiO
the hole trapping sites (i.e. oxygen vacancies) [28,29]. The EPR of TiO
SnO resembled that of TiO , showing all the three g , g and g reso-
O
2
2
1
2
3
=
2
1
2
(lattice Ti3 ), while g
+
is assigned to
3
2
2
-
Adsorp TiO -SnO2
2
TiO -SnO2
2
2
2
1
2
3
Desorp
nance lines, thus indicating the high concentration amount of oxygen
vacancies.
9
.07 nm
Fig. 2 show the TEM and HRTEM micrographs images of the heter-
ostructure TiO
agglomerated densely packed nanoparticles (NPs) with non-uniform
shapes and sizes. For TiO , the morphology shows the formation of
2 2 2
-SnO based catalysts. The SnO showed to compose of
2
densely packed agglomerates of short amorphous rod-like particles with
a diameter range of about 4–5 nm and the length ranges between 8 and
2
Fig. 3. N isotherms and BJH pore size distribution plots of the catalysts.
3

P.M. Malibo et al.
Inorganic Chemistry Communications 129 (2021) 108637
(
c)120
00
1
8
6
4
2
0
0
0
0
0
SnO2
TiO2
TiO -SnO
2 2
Catalysts
Fig. 4. Catalytic activity screening results of TiO
2
-SnO
2
catalysts in the oxidation of furfural. (a) Selectivity; (b) Yield; and (c) H
2
O
2
utilization efficiency. Reaction
◦
conditions: furfural (5 mmol), 30% (aq)
H
O
2 2
(25 mmol), catalyst (100 mg), T = 60 C, t = 24 h, 10 ml co-solvent of H
2 3
O (5 ml) and CH CN (5 ml) was used. Symbols:
OA = oxalic acid; MA = maleic acid; SA = succinic acid; FA = furoic acid.
1
0 nm. The TiO
2
-SnO
2
catalyst shows the morphology similar to that of
which is narrow and uniformly distributed within the range 46 nm with
the average centred at 9.07 nm. SnO broad distributed pores sizes
SnO NPs but with less agglomeration, which corroborate its high sur-
2
2
face area and porosity compared to the single oxides (Table 1). The
selected area electron diffraction (SAED) patterns are characterized by
the diffraction rings, which indicate the polycrystalline nature of the
centred at both 3.53 nm and 18.55 nm while of TiO
420 nm.
2
are in the range of
Fig. 4 shows the comparison results of the catalytic performance of
the TiO -SnO catalysts in the furfural oxidation reaction. SnO obtained
the furfural conversion of 18.3% and maleic acid selectivity of 14.1%
while TiO afforded 23.7% furfural conversion and maleic acid selec-
tivity of 63.4% (Fig. 4a). Further, SnO showed high selectivity of 83.5%
towards 2-furoic acid while TiO did not (20.7%). Both SnO and TiO
also showed the formation of fumaric acid, oxalic acid, and succinic acid
at low selectivity. The TiO -SnO catalyst exhibited a markedly
improved catalytic activity with furfural conversion of 96.2% when
compared to SnO and TiO . The obtained high furoic acid selectivity of
83.5% for SnO decreased significantly for TiO -SnO to 5.1%. TiO
SnO obtained a maleic acid formation yield of 63.8% at selectivity of
66.3% (Fig. 4a). Fig. 4c shows the H consumption amounts of the
respective SnO , TiO and TiO -SnO catalysts in the oxidation reaction
of furfural. The high-consumed amount of H was obtained with the
TiO -SnO catalyst, which also gave the highest conversion amount of
furfural, thus yield of maleic acid. This high activity of TiO -SnO is due
to its enhanced redox activity manifested by the modified structure in-
teractions of the two metal oxides. The participation of H in furfural
heterostructure catalyst. The diffraction rings of SnO
catalysts comprises discrete spots, whereas those of TiO
diffused which indicate amorphous phase. The three most inner indi-
cated SAED rings for SnO and TiO -SnO catalysts correspond to the
respective (1 1 0), (1 0 1) and (2 1 1) planes of the SnO tetragonal
phase, which are in agreement with the XRD (Fig. 1a) showing pre-
dominantly the SnO reflection peaks. In addition to the three rings,
there is a fourth ring corresponding to the SnO (1 1 2) plane for SnO
and SnO (3 0 1) plane for TiO -SnO . The diffraction ring for anatase (1
2) was observed for TiO -SnO . The indexed rings for TiO correspond
to the respective (2 0 0), (2 0 4) and (2 1 5) planes of anatase.
2
and TiO
2
-SnO
2
2
2
2
2
are more
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
2
2
2
2
2
2
2
-
2
2
2
2
BET surface areas of 13.2 m /g, 7.6 m /g and 72.6 m /g were
measured for TiO , TiO and TiO -SnO , respectively (Table 1). The pore
diameter of TiO -SnO is 11.0 nm while of TiO and SnO are respec-
tively 9.0 nm and 9.5 nm. Further, TiO -SnO shows higher pore volume
of 0.20 cm /g compared to 0.031 and 0.017 cm /g for SnO
respectively. The high surface area of TiO -SnO correspond to the
2 2
O
2
2
2
2
2
2
2
2
2
2
2
2
2 2
O
2
2
2
2
3
3
2
and TiO
2
,
2
2
2
2
formation of nanosized particles comparable to single oxides as evi-
denced by the TEM images with size diameter of 10–20 nm (Fig. 2).
The adsorption–desorption isotherms and pore size distribution plots
2 2
O
oxidation reaction can be related to the typical Fenton-like driven cat-
alytic radical’s species, which form the active oxygen species such as
hydroxyl following the involvement of Ti4 /Ti redox cycle mecha-
+
3+
2 2
of the catalysts are presented in Fig. 3. The TiO -SnO catalysts shows
the type IV with classified H2 hysteresis loops characteristic of meso-
porous materials [30]. This type of isotherm is an indication of the
particles forming agglomerates arranged in a uniform way that have
pores with narrow mouths and relatively uniform channel-like pores.
nism outlined below:
Ti3 + H
+
O
→ Ti + HO + OH
4+
•
ꢀ
+
(1)
(2)
(3)
2
2
2
4+
3+
•
Ti + H
O
2
→ Ti + HOO + H
The mesoporous character of the heterostructured the TiO
2 2
-SnO com-
Ti3 + HO → Ti + OHꢀ
+
•
4+
posite nanocatalyst was confirmed further by the pore size distribution,
4

P.M. Malibo et al.
Inorganic Chemistry Communications 129 (2021) 108637
2 2
furfural to maleic acid. The structural interface of TiO and SnO dis-
1
20
00
played a synergistic catalytic activity enhancement due to the modified
electronic structures, which is manifested by effective redox cycles and
Furfural MA OA FumA SA FA Other
1
2 2
oxygen vacancies than the single oxide counterpart. The TiO -SnO
catalyst gave a furfural conversion of 96.2% to obtain maleic acid yield
of 63.8% at 66.3% selectivity. Characterisation results showed that the
enhanced catalytic performance was due to the structure effect from the
8
6
4
2
0
0
0
0
0
oxygen vacancies, and improved redox reactive species. Both TiO
2
and
SnO are cheap and abundantly available metals, which can present an
2
economical catalytic process for upgrade of biomass-derived furfural
into renewable biobased maleic acid synthesis.
CRediT authorship contribution statement
Petrus M. Malibo: Formal analysis, Methodology, Investigation,
Visualization, Writing - original draft. Peter R. Makgwane: Conceptu-
alization, Investigation, Validation, Resources, Funding acquisition,
Supervision, Project administration, Writing - review & editing. Pris-
cilla G.L. Baker: Conceptualization, Writing - review & editing,
Supervision.
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Reaction cycles
2 2
Fig. 5. Recyclability performance test of TiO -SnO catalyst in furfural oxida-
tion reaction. Reaction conditions: furfural (5 mmol), 30% (aq)
H
2
O
2
(25 mmol),
◦
catalyst (100 mg), T = 60 C, t = 24 h, 10 ml co-solvent of H
2
O (5 ml) and
Declaration of Competing Interest
CH
3
CN (5 ml) was used. Symbols: OA = oxalic acid; MA = maleic acid; SA =
succinic acid; FA = furoic acid.
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.
is decomposed by Ti4 /Ti of the TiO
+ 3+
2 2 2 2
In first step, H O -SnO
catalyst to form highly reactive hydroxyl species during the reaction
based on Eqs. (1) and (2). The formed hydroxyl species are reactive to
activate the furfural molecule to undergo several functional groups re-
arrangements, which include the subsequent trapping of the oxygen
species from the available hydroxyl source to stable oxygenated mole-
Acknowledgements
CSIR-SRP thematic (grant number T#18/2019-2020) is thanked for
the financial support. The financial support for analysis of XPS samples
from Dr M.M. Mphahlele-Makgwane (University of Limpopo) is
acknowledged.
cules as products. The XPS (Fig. S2) and EPR (Fig. 1e) showed the TiO
SnO catalyst to possess high amount of surface oxygen defects than
TiO and SnO , which was beneficial for the effective furfural conver-
sions and at high preserved maleic acid selectivity and yields. The redox
2
-
2
2
2
Appendix A. Supplementary material
4
+
3+
activity of surface exposed Ti /Ti has also showed previously to be
effective to facilitate the enhanced electrons transfer mobility in H
mediated oxidation reactions [31]. Further, the high surface area of
TiO -SnO could also effect a significant textural property on catalytic
performance (Table 1). According to XPS, the interfaced TiO -SnO exist
2 2
O
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.inoche.2021.108637.
2
2
2
2
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