Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst

Article abstract of DOI:10.1007/s10562-020-03499-z

Abstract: In the present investigation, photocatalytic conversion of xylose by Copper (Cu) doped Zinc oxide (ZnO) was investigated under Ultraviolet Light emitting diode (UVA-LED) illumination. Photocatalysts were synthesized successfully by chemical prec

Full text of DOI:10.1007/s10562-020-03499-z

Catalysis Letters
https://doi.org/10.1007/s10562-020-03499-z
Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc
Oxide Catalyst
B. Rohini1,2 · H. Umesh Hebbar1,2
Received: 9 September 2020 / Accepted: 14 December 2020
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
In the present investigation, photocatalytic conversion of xylose by Copper (Cu) doped Zinc oxide (ZnO) was investigated
under Ultraviolet Light emitting diode (UVA-LED) illumination. Photocatalysts were synthesized successfully by chemical
precipitation method. The synergistic efect of 5 wt% Cu doped ZnO and addition of glycerol as oxygen scavenger improved
conversion. The results from our study showed that %conversion of xylose, glycerol are 33.72%, 33.61% respectively and %
product yield of 88.79% of Dihydroxyacetone(DHA), 19.87% of xylitol and 13.29% of erythritol were achieved when 1.66 g/L
of catalyst were used in ambient conditions under 7 h of UVA-LED illumination. The varied temperature to 50 2 °C had
decreased efect on the product yield when compared to that of the reaction carried out at 30 2 °C. High Resolution Mass
spectrometry results confrmed the presence of the products xylitol, erythritol and DHA formed during the course of the
photocatalytic reaction.
Graphic Abstract
Keywords Biomass conversion · Photocatalysis · UVA-LED · Xylose · Cu doped ZnO · Glycerol · Xylitol
1 Introduction
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s1056
2-020-03499-z.
Biomass valorisation for the production of valuable
chemicals have paved the way to combat depletion of fos-
sil resources. The utilisation of naturally occurring sugar
such as xylose for producing ethanol, furfural, and xylitol
* H. Umesh Hebbar
hebbar@cftri.res.in
Extended author information available on the last page of the article
Vol.:(0123456789)
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B. Rohini, H. U. Hebbar
through catalytic conversion processes has been the topic
of current research [1]. About 20–30 wt% of lignocellulosic
biomass consists of hemicellulose, which mainly compris-
ing of d-xylose residues [2]. Xylose is a pentose sugar (C5)
with molecular formula C5H10O5 is often used as a food
sweetener [3]. Xylose dehydrates to furfural in the pres-
ence of diferent types of acid catalyst, either Brønsted or
Lewis catalyst under high temperature conditions generally
between 140 and 240 °C [4]. However, the above process
uses high-pressure conditions or microwave radiations,
which causes structural instability in catalysts and also its
leaching, resulting in lower recovery for reuse [5]. Industri-
ally, Xylitol is produced by catalytic hydrogenation in the
presence of a noble metal-free catalyst, RANEY® nickel
at 80–130 °C under 40–70 bar of hydrogen (H2). Xylose
reduces to xylitol between 100 and 125 °C (lower than the
melting point of xylose) in the presence of the noble metal
based catalyst such as ruthenium (Ru), rhodium (Rh) and
platinum (Pt) [6–9]. Another study [10] reported that Ru
appeared as the most efcient catalyst for selectively con-
verting xylose to xylitol in the presence of rutile phase TiO2
supported Ru (1 wt%).
photocatalytic semiconductor based particles have found
its application in the feld of biomass conversion. ZnO is
an n-type wide band gap semiconductor with high photo-
catalytic activity, high thermal and mechanical stability,
and non-toxic nature. Numerous nanoscale morphologies
including nanoparticles, thin flms, dumbbell-shaped crys-
tals, micro-/nano-rods, nanowires, and nanopetals have been
used to assess the photocatalytic properties of ZnO [17].
However, the use of ZnO particles as a photocatalyst has
some limitations such as fast recombination of electron–hole
pairs and its susceptibility to deterioration upon exposure
to UV irradiation in aqueous solution, a phenomenon usu-
ally referred to as photocorrosion [18]. In order to suppress
the photocorrosion efect and to increase its photocatalytic
activity, various techniques such as coating with noble met-
als, doping with metals and creating composites with other
semiconductors have been practiced [19]. ZnO particles
have been doped with diferent metals such as Cu, Al, Ag,
Sn, Ga for improving photocatalytic activity [20]. Cu–ZnO
composites have wide applications in catalytic conversion
of CO2 or CO to methanol, hydrogenation/dehydroxylation
of biomass (fructose, sorbitol, glycerol or furfural) to diols,
photocatalytic H2 production, and methanol steam reforming
[21]. These catalytic hydrogenation reactions require highly
fammable molecular hydrogen gas that is obtained from
petrochemical sources. The present challenge is to replace
hazardous hydrogen gas with an alternative safer, cleaner
source for hydrogenation reactions. This could be achieved
through utilisation of solvents such as glycerol and isopro-
panol as hydrogen donors. Studies conducted by Vaiano
et al. [22], Díaz-Álvarez and Cadierno [23] and Natarajan
et al. [24] prove that hydrogenation reactions can be carried
out with the use of greener solvents by reducing the need of
petroleum based hydrogen source. Also, most of the exist-
ing photocatalytic studies and applications have used Xenon
lamps, mercury lamps, and Iron-halide lamps as light source.
However, the use of these light sources has limits due to
lack of achieving the path of sustainability and eco-friendly
sources [25]. Light-emitting diodes (LEDs) recently became
a promising alternative light source for photocatalytic appli-
cations for their high energy efciency, long lifetime, com-
pact size and DC power supply availability [3, 26].
Nevertheless, in comparison with the use of Ni based
catalyst, use of highly precious noble metals may not be the
desirable choice for large scale applications. An activated
carbon (AC) supported Ni–Re bimetallic catalysts (Ni–Re/
AC) were used to hydrogenate xylose and hemicellulosic
hydrolysate into xylitol under mild reaction conditions
[11]. Also, Ru decorated carbon foam (Ru/CF) was used
as a catalyst to convert xylose to xylitol in the presence of
external hydrogen gas at 40–60 bar and high temperature of
100–120 °C to obtain maximum yield. The studies discussed
above involves the processes that requires high temperature,
H2 atmosphere which is energy consuming, costly, tedious
and not so environmental friendly [12]. Photocatalysis is
reported to be one such technology which ofers sustainable
route for the production of valuable compounds. Photoca-
talysis has been considered as a promising alternative by
carrying out the reactions in ambient and mild conditions
[13]. “Photocatalysis” is the term to describe the accelera-
tion of chemical reactions by photo-induced electron/hole,
in the presence of a photocatalyst. The process is considered
as clean, energy-saving, technologically simple, ecologi-
cally benign and low cost ofering the possibility of extend-
ing the spectrum of applications to a variety of processes,
including oxidations and oxidative cleavages, reductions,
isomerizations, substitutions, condensations, and polymeri-
zations [14]. Photocatalysis has been extensively studied for
contaminants remediation, fne chemical synthesis, proton
reduction, carbon dioxide (CO2) reduction [15]. In recent
decades, Photocatalysis fnds its application in the conver-
sion of biomass to high value-added products by replacing
the conventional method of production [16]. Currently, the
Considering the growing interest in the area of valorisa-
tion of biomass and important fndings in recent years on
photocatalytic conversions, an attempt has been made to
investigate photocatalytic conversion of xylose. Cu doped
ZnO nanoparticles, activated using UVA-LED are used as
photocatalyst. The reactions are carried out under ambient
conditions and by utilizing glycerol as hydrogen donor to
avoid usage of petroleum based hydrogen gas. The study
is focused on analysing the efect of varying catalyst load,
addition of glycerol and reaction temperature on efciency
of conversion of xylose.
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Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst
of the reactor. The reactor contained port for purging nitro-
2 Materials and Methods
gen gas used to remove dissolved oxygen inside the reactor
and a sampling port fxed with self-sealing septa to withdraw
the sample from the syringe. To prevent loss of illumination,
the reactor was covered with aluminum foil. The reactor is
placed on the magnetic stirrer to achieve uniform mixing up
of the solution.
2.1 Chemicals
Zinc acetate dihydrate (Zn(CH3·COO)2·2H2O), copper chlo-
ride (CuCl2·2H2O), diethylene glycol, sodium hydroxide
(NaOH) were purchased from Merck Private Ltd. (Mumbai,
India). d-xylose (99%), xylitol (99%), glycerol anhydrous
(C3H8O3), dihydroxyacetone (DHA), and erythritol (99%),
solvents for UHPLC analysis such as acetonitrile, water,
Triethylamine (TEA) were purchased from Sisco Research
Laboratories Pvt. Ltd. (Mumbai, India). UVA-LED strip
were purchased from Probots Techno Solutions (Bengaluru,
India).
3.3 Photocatalyst Characterization
3.3.1 UV–Vis Spectroscopy
The UV–visible optical absorption spectrum of the undoped
ZnO and Cu doped ZnO have been carried out at room tem-
perature using UV–Vis spectrophotometer (model Shimadzu
UV 1601, Japan) by measuring the wavelength from 200 to
700 nm. The energy band gap for both undoped ZnO and Cu
doped ZnO has been calculated using the Tauc relation [28].
3 Methods
A(E − Eg)p
3.1 Photocatalyst Preparation
ꢀ(E) =
(1)
E
Pure Zinc oxide (ZnO) nanoparticles were purchased com-
mercially from Sisco Research Laboratories Pvt. Ltd. (Mum-
bai, India). Copper (Cu) doped ZnO nanoparticles were syn-
thesized by wet chemical precipitation method as described
by Rahmati et al. [27]. Briefy, Zn (CH3·COO)2·2H2O and
CuCl2·2H2O were mixed stoichimetrically to obtain 5 wt%
Cu doped ZnO. The solution was stirred for 10–15 min to
which NaOH solution was added drop wise to obtain the pre-
cipitate. Then to the precipitated solution diethylene glycol
was added and continued stirring for 10–15 min. Bluish-
white sol–gel precipitated particles were separated by cen-
trifugation @7500 rpm for 15 min. The collected particles
were washed with distilled water 3–4 times followed by
washing with ethanol for 2–3 times to remove any impuri-
ties. The particles were dried in hot air oven @ 100 °C for
4 h and further subjected to calcination in a programmed
mufe furnace @ 500 °C for 1 h. The obtained particles
were used for further studies.
where ‘A’ is a constant, ‘Eg’ is optical energy band gap of
the material, ‘p’ is exponent which depends upon the kind of
transition (1/2, 2, 3/2) for direct, indirect and direct forbid-
den transitions.
3.3.2 X‑Ray Difraction (XRD) Analysis
The crystalline structure of undoped and Cu doped ZnO par-
ticles were investigated using X-ray difractometer (Rigaku,
Japan). The X-ray difraction spectrum was recorded from
Cu-kα radiation at a wavelength of 1.54 Å. The difracto-
gram was collected in refection mode by scanning the 2theta
range from 1° to 80° with a step size of 1°/min at room
temperature. The average crystallite size is estimated by the
following Debye-Scherer formula:
0.9ꢀ
D =
(2)
ꢁcosꢂ
where, ‘D’ is the crystallite size (nm), ‘λ’ is wavelength of
X-ray used (nm), ‘β’ is full width half maximum (FWHM in
radians), ‘θ’ is Bragg’s difraction angle (in degree).
3.2 Development of Lab Scale Photocatalytic
Reactor
A laboratory scale experimental set-up was developed from
three necked glass rectangular fat bottom fask of 500 mL
capacity and are shown in Fig. 1a. UVA-LED strip was used
as light source for illumination. The wavelength emission
range of UVA-LED strip used in our study were analysed
using Spectral radiometer (Spectral Evolution SR-3500,
Department of Earth Science, University of Mysore). To
achieve uniform illumination and as much efective as possi-
ble, the LED strip was rolled up around the external surface
3.3.3 Scanning Electron Microscopy with Energy Dispersive
Spectrum (SEM–EDS)
The structure and morphology of undoped ZnO and Cu
doped ZnO nanoparticles were examined through Scanning
Electron Microscope (SEM) (Leo 435 VP, Leo Electronic
systems, Cambridge, UK). Also, the elemental compositions
of the nanoparticles were analyzed using SEM equipped
with Energy Dispersive Spectrum (EDS/EDAX).
1 3

B. Rohini, H. U. Hebbar
Fig. 1 a Schematic representation of photocatalytic reactor; b the wavelength range of UVA-LED strip measured from spectral radiometer
1 3

Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst
3.3.4 FTIR Studies
performed using an ACQUITY BEH amide HILIC col-
umn (2.1 mm × 100 mm × 1.7 m, Waters, Mildford, MA,
USA). The mobile phase consisted of gradient solvents A
and B. Solvent A contained 80% acetonitrile: 20% water:
0.2% TEA and solvent B contained 30% acetonitrile: 70%
water: 0.2% TEA. The injection volume used for stand-
ards and samples were 1 µL and were performed with a
fow rate of 0.17 mL/min for 18 min with gradient elution:
0 min (0–100% A), 0–10 min (40–100% A, 0–60% B), and
10–18 min (100% A).
FTIR spectra of particles were recorded in the range
4000–400 cm−1, using FTIR-8400S spectrometer (Shi-
madzu, IR solution 1.30, Shimadzu Corporation, Japan).
3.4 Photocatalytic Reaction Studies
The reactions were carried out using 5 g/L of initial xylose
concentration, 1.66 g/L of catalyst, 300 mL of reaction
volume, UVA-LED illumination. The reactor was purged
with nitrogen gas for 10–15 min before the start of the
reaction and the solution was kept under constant stirring
condition @ 300 rpm for about 7 h. The liquid samples
were drawn hourly and fltered through 0.22 µm syringe
flter to remove the catalyst from the liquid sample and
subjected for further analysis. Photocatalytic reactions
were performed to evaluate the conversion of xylose
into value-added products such as xylitol, erythritol. The
degree of conversion of xylose and glycerol were calcu-
lated as below and presented as percentage conversion
3.5.2 High Resolution Mass Spectroscopy (HRMS)
High resolution mass spectroscopy studies were carried
out for the identifcation of reaction products. Triple-TOF
(Time of fight) high resolution mass spectrometer was used
to analyze the samples through direct infusion. To assist the
ionization and desolation, the liquid samples were diluted
with a 1:1 mixture of acetonitrile and water. The full scan
mass spectra were acquired in negative ionization mode with
collision energy: − 10 eV, mass scan range of m/z 30–500.
The operating conditions maintained were: vacuum gauge
pressure: 3 × 10−5 Torr, source temperature: 400 °C set
point, tray temperature: 10 °C, injection volume: 60 μL, nee-
dle volume: 7 μL. The data were analysed with Peak View
2.1 Software (AB SCIEX Triple TOF 5600, Singapore),
equipped with MasterView™ (Version 1.0, AB SCIEX).
(initial xylose − xylose at time t)
Conversion of Xylose (%) =
× 100
initial xylose
(initial glycerol − glycerol at time t)
Conversion of Glycerol (%) =
× 100
initial glycerol
(concentration of product at time t)
Product yield (%) =
× 100.
initial concentration of reactant
4 Results and Discussions
3.5 Liquid Sample Analysis
4.1 Spectral Analysis of Light Emitting Diode
All liquid samples were analysed and quantifed using
Ultra High Performance Liquid Chromatography
(UHPLC) equipped with Evaporative Light Scattering
Detector (ELSD) and presence of reaction products were
confrmed through High Resolution Mass Spectrometry
(HRMS).
The wavelength pattern is shown in the Fig. 1b. A UVA-
LED strip containing 200 LEDs with light intensity of 35
mW/cm2 was used as light source. The wavelength was
found to be in the range of 380–400 nm which lies in the
UVA region from the electromagnetic spectrum.
4.2 Characterization Studies
3.5.1 Ultra High Performance Liquid Chromatography
with Evaporative Light Scattering Detector
(UHPLC‑ELSD)
4.2.1 UV–Vis Absorption Spectroscopy
The UV–visible optical absorption spectra (300–700 nm)
obtained at room temperature for undoped ZnO and Cu
doped ZnO nanoparticles are presented in Figure S1a. The
maximum absorption wavelength of undoped ZnO shifted to
slightly lower wavelength when doped with Cu. This shift in
lower end is referred to blue shift and the reason attributed to
this is the increase of lattice distortion. The similar observa-
tion of wavelength shift to lower end was reported by Sindhu
et al. [28] and Ashokkumar and Muthukumaran [29]. The
The chromatographic analysis were performed using a
ACQUITY UHPLC system (Waters, Milford, USA) con-
sisting of a quaternary gradient pump (H-Class), a vacuum
degasser, a sample manager with thermostat and Flow-
Through Needle (FTN), a column compartment equipped
with preheater and an evaporative light scattering detec-
tor (ELSD). Empower Pro Software, version3 were
employed for controlling the system. The separations were
1 3

B. Rohini, H. U. Hebbar
graph of α(E)2 v/s E was plotted and shown in the Figs. S1b
& S2c for undoped and 5 wt% Cu doped ZnO, respectively.
The extrapolation of the straight line to the axis of energy
(E) gives the band gap energy value of the material. The
energy band gap value obtained for undoped ZnO and Cu
doped ZnO are 3.3 eV and 3.4 eV, respectively. This increase
in the energy band gap is in correlation with the blue shift
of absorption wavelength. Similar increase in energy band
gap were reported by Ashokkumar and Muthukumaran [29]
in their studies when the Cu concentration was increased
from 0.01 to 0.5.
corresponds to miller indices (1 0 0), (0 0 2), (1 0 1), (1 0
2), (1 1 0), (1 0 3), (2 0 0), respectively. Another additional
peak with very low intensity at 38.7° was observed in Cu
doped ZnO, which corresponds to (1 1 1) plane exhibiting
monoclinic structure of CuO. The difraction peak of CuO
were consistent with the values, as previously reported by
Chen et al. [31] and Jiang et al. [32]. From the study Choi
et al. [33] reported showed that Cu doped ZnO, prepared by
conventional hydrothermal method had a similar plane of
(1 1 1) and peak at 38.7° corresponding to CuO, when Cu
loading were increased from 1.0 to 5.0 mol%.
The crystallite size of undoped ZnO and Cu doped ZnO
were calculated by using the Eq. (2), and values were found
to be 27.78 nm and 31.8 nm, respectively. From the XRD
spectra, by careful observation of peak from 31.0° to 38.0°
reveals that the slight shift in the 2 theta value and broad-
ening of difraction peak at 36° indicates that Cu has been
incorporated into the ZnO lattice with no change in crystal
lattice.
4.2.2 XRD Analysis
XRD analyses were carried out to identify the crystal struc-
ture, lattice parameters and crystal size of both undoped and
Cu doped ZnO. The XRD spectra of undoped and Cu doped
ZnO are depicted in Fig. S2. From the XRD spectra, it was
observed that sharp and highly difracted peaks implies par-
ticles exhibit good crystalline nature. “High Xpert score”
software was employed to study the XRD patterns and the
database available in the software showed same XRD pattern
for both undoped and Cu doped ZnO with the reference code
of (01-097-0207), indicating the Cu doping on ZnO particles
have been successfully achieved.
4.2.3 SEM and EDS Studies
The surface morphology of undoped ZnO and Cu doped
ZnO were analysed through Scanning Electron Micros-
copy (SEM). From the Fig. 2a, b it can be seen that the
undoped ZnO particles exhibited needle like structure. Since
the particles were agglomerated, clear distinctive image of
undoped ZnO nanoparticles could not be observed. In the
Fig. 2c, d, it is evident that the particles were in hexagonal
structure which corroborates with the results obtained from
XRD analysis. Also, the elemental composition of undoped
and Cu doped ZnO were studied and data are shown in the
The results obtained from the database showed parti-
cles exhibited hexagonal wurtzite phase. Also, the particles
belonged to P63mc space group with JCPDS card (36-1451).
Similar observation was reported by Sakai et al. [30]. The
characteristics peak of undoped and Cu doped ZnO were
observed at an angle 2 theta i.e. at 31.79°, 34.45°, 36.25°,
47.53°, 56.51°, 62.84°, 66.50°, 67.87°, 69.06° which
Fig. 2 SEM images a, b undoped ZnO; c, d 5 wt% Cu doped ZnO EDS of e undoped ZnO f 5 wt% Cu doped ZnO
1 3

Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst
Fig. 2e and f, respectively The percentage composition
measured in Energy dispersive spectrum (EDS) studies con-
frmed the presence of main constituents Zn, O in undoped
ZnO and Zn, O, Cu in case of Cu doped ZnO, indicating that
there were no presence of any impurities in the particles.
Also, the amount of Cu added stoichimetrically during the
preparation of Cu doped ZnO were comparable to the atomic
weight % obtained from EDS analysis.
Cu doped ZnO nanoparticles. The results obtained from
our study were comparable with that obtained by Payorm-
horm et al. [36] where they achieved 6.45% of xylitol from
photocatalytic conversion of glucose under UV irradia-
tion (λ = 365 nm) by surfactant assisted fabricated TiO2
as catalyst.
In our study, the reaction was carried out without any
supply of external hydrogen source and using glycerol
as sole hydrogen donor which further undergoes oxida-
tion reaction. The results of our study is supported by
the observations of [22], who proved that it is possible to
produce hydrogen from glycerol aqueous solution using
Cu doped ZnO as catalyst without the supply of external
hydrogen gas. Also, Yi and Zhang [21] produced xylitol
from hemicellulose in one-pot reaction conditions using
Ru/C catalyst using isopropanol as hydrogen donor. The
authors achieved more than 80% xylitol yield @140 °C,
within 3 h. In the present study, in order to study the efect
of addition of glycerol, reaction was carried out without
the presence of xylose by Cu doped ZnO nanoparticles and
the results are shown in the Fig. 3c. The results revealed
that only DHA were formed as the reaction product when
glycerol alone was used for reaction and no formation of
xylitol and erythritol was observed. Díaz-Álvarez and
Cadierno [23] reported that glycerol could be efciently
used as solvent and hydrogen donor in metal-catalysed
transfer hydrogenation reactions. Their study showed that
glycerol undergone oxidation reaction to produce DHA
and release hydrogen molecule. Another study by Estah-
banati et al. [37] proved that glycerol could be converted
to hydrogen only when TiO2 nanoparticles were loaded
with Pt metal. These evidences prove that the hydrogen
donor and doping are crucial parameters for the photo-
catalytic conversion of xylose into valuable compounds
such as xylitol, erythritol and, also supports the results
achieved in our study.
4.2.4 FTIR Analysis
Fourier transform infrared spectroscopy (FTIR) spectrum
was observed to investigate the chemical composition of the
samples. The spectrum was obtained in the range from 400
to 4000 cm−1 and shown in the Fig. S3a. The major peaks
were observed in the range 400–600 cm−1 wavenumber
(shown in Fig. S3b) which is characteristic peak of metal
oxide vibrations [34]. The peak at wavenumber 554 cm−1
and 412 cm−1 corresponds to Zn–O stretching of ZnO and
the wavenumber at 600 cm−1 corresponds to Cu ions incor-
poration [35]. The results are in correlation with the UV–Vis
and XRD analysis proving that Cu doping on ZnO surface
lattice has been successfully achieved.
4.3 Photocatalytic Studies
4.3.1 Synergistic Efect of Cu Doping and Addition
of Glycerol on Photocatalytic Conversion of Xylose
The graph in the Fig. 3a shows degree of conversion of
xylose by undoped ZnO and 5 wt% Cu doped ZnO nanopar-
ticles. About 8% of xylose conversion was achieved within
7 h when 5 wt% Cu doped ZnO were used for the reaction
while the undoped ZnO did not have any efect on the con-
version of xylose. The reason could be due to the photocor-
rosion efect of undoped ZnO upon continuous exposure to
UVA light [18]. From the results it is evident that doping has
an efect on the photocatalytic conversion of xylose which
could be due to the prevention of recombination of elec-
tron–hole pair and suppression of photocorrosion efect of
ZnO particles [19] resulting in improving the photocatalytic
activity of Cu doped ZnO.
Figure 4 shows the possible schematic representation
of the reaction and the mechanism of the photocatalytic
conversion of xylose in the presence of glycerol and Cu
doped ZnO nanoparticles are explained as follows:- (1)
when the UV-LED (380–400 nm) is incident on surface of
Cu doped ZnO creates electrons (e−) and holes (h+) which
migrate to valence band and conduction band inside the
ZnO nanoparticles (2) the generated electrons and holes
are separated; electrons are trapped by Cu particles by
preventing recombination of electron–hole pair (3) simul-
taneous oxidation of glycerol to DHA and reduction of
xylose to xylitol and erythritol are formed.
Further, experiments were carried out using Cu doped
ZnO nanoparticles with the addition of glycerol, as oxygen
scavenger. The Fig. 3b shows the degree of conversion of
xylose when both Cu doped ZnO nanoparticles and glyc-
erol were used for the photocatalytic reaction. The results
showed 21.63% conversion of xylose and 21.84% of glyc-
erol along with the formation of valuable products such
as 4% xylitol, 50% of DHA and 6.3% of erythritol. The
considerable increase in the degree conversion of xylose
could be attributed to the synergistic efect of glycerol and
4.3.2 Efect of Catalyst Dosage
In order to study the efect of dosage of catalyst, the cata-
lyst load was varied and tested at three levels, i.e., 0.83 g/L,
1 3

B. Rohini, H. U. Hebbar
Fig. 3 Photocatalytic conver-
sion of xylose: a by undoped
ZnO and 5 wt% Cu doped
ZnO nanoparticle; b showing
synergistic efect of Cu doped
ZnO nanoparticles; c efect of
addition of glycerol without the
presence of xylose by Cu doped
ZnO nanoparticles
1 3

Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst
Fig. 4 Schematic representation
of possible mechanism of pho-
tocatalytic conversion of xylose
holes which could be observed with the increase in degree
of conversion of xylose. But on further increase in catalyst
dosage to 3.3 g/L, the conversion of both xylose and glycerol
decreased. The decrease in the photocatalytic activity may
be related to increase in opacity of aqueous reaction solution
making the penetration of UVA-LEDs light gradually dif-
fcult [22], thereby reducing the formation of electrons and
holes for the reaction to carry out. Similar trend of decrease
in xylose conversion and xylitol yield were reported by Liu
et al. [38] when Ru nanoparticles loading was increased due
to aggregation of Ru nanopartilces. At the optimum cata-
lyst load condition (1.66 g/L), xylose and glycerol conver-
sion were 33.72% and 33.61%, respectively, while product
yield was 88.79% of DHA, 19.87% of xylitol and 13.29%
of erythritol.
Fig. 5 Efect of catalyst load on conversion and yield
4.3.3 Efect of Reaction Temperature
1.66 g/L and 3.3 g/L, keeping other parameters such as
initial xylose concentration (5 g/L), glycerol concentration
(3.3 g/L) and duration of UVA-LED illumination (7 h), the
constant.
The efect of temperature on conversion of xylose was stud-
ied by performing the reaction at 30 2 °C and 50 2 °C.
The above temperature range was selected, as the recom-
mended limit of operation for UVA-LED was < 60 °C (as
per manufacturer). During the course of the reaction, rise
in temperature was observed by nearly 5 °C. This is due to
the heat generation due to continuous exposure of UVA-
LED illumination for a long period (7 h) and mild stirring
during this period. The other conditions of reaction were:
initial xylose concentration of 5 g/L, 1.66 g/L of catalyst
load, 3.3 g/L glycerol.
From the Fig. 5 it can be observed that at 0.83 g/L of cata-
lyst load degree of conversion of xylose and glycerol were
21.63% and 21.84%, respectively. As the catalyst load was
increased to 1.66 g/L, the conversion also increased leading
to increase in product yield. However, further increase to
3.3 g/L of catalyst load the degree of conversion showed
decreasing trend, though the value was higher than that
obtained at 0.83 g/L. The above trend observed in our study
could be attributed to the availability of more surface active
site when catalyst dosage is increased from 0.83 to 1.66 g/L,
which also resulted in increase in formation of electrons and
From the Fig. 6, it is observed that when the reaction tem-
perature was increased to 50 2 °C, the percentage yield of
reaction products, namely, DHA, xylitol, erythritol decreased
1 3

B. Rohini, H. U. Hebbar
4.3.4 HRMS Studies
The products obtained under optimum reaction conditions
of 1.66 g/L of catalyst dose and 30 2 °C reaction tem-
perature were further characterised using HRMS technique
to confrm the products. The total MS spectra obtained
is shown in the Fig. S4. The presence of DHA, eryth-
ritol and xylitol were identifed and confrmed based on
their respective mass to charge (m/z) ratio of 89.0248,
121.0422 and 151.0602. Further the individual products
were fragmented to match fragmented ions with the avail-
able database Pubchem and showed exact fragments which
are detailed in Table 1.
Fig. 6 Efect of varying reaction temperature
5 Conclusion
its yield by 20.44%, 6.49%, 8.765% respectively than those
formed at 30 2 °C. Till date, there are no studies reported
on obtaining xylitol, erythritol and DHA under mild tempera-
ture using Cu doped ZnO nanoparticles as catalyst. However,
recent studies reported shows xylitol formation only at high
temperature and using noble metal based nanoparticles as
catalyst. Few such studies reported are by Mishra et al. [39]
when Ru supported zeolite Y catalyst was used for hydrogena-
tion of xylose to xylitol by varying temperature from 100 to
140 °C, the results showed that increase in temperature of the
reaction decreased the xylitol selectivity due to formation
of by-product such as arabitol though the xylose conversion
increased. Another study by Liu et al. [40] reported that when
reaction temperature was increased from 50 to 190 °C and at
hydrogen pressure of 1 atm for 148 h of reaction time, achieved
high selectivity in xylose to xylitol conversion at 50 °C using
Ru nanoparticles combined with MOF (ZIF-67) catalyst. In
our study, we have achieved the formation of xylitol, DHA
and erythritol even at 30 2 °C which is nearly ambient tem-
perature condition thereby minimising undesirable product
formation.
The present work explored, the possibility of obtaining
xylitol from photocatalytic conversion of xylose with Cu
doped ZnO as photocatalyst under UVA-LED illumina-
tion. Under optimized conditions of photocatalytic reac-
tion, 88.79% of DHA, 19.87% of xylitol and 13.29% of
erythritol were obtained. Our study showed that the addi-
tion of glycerol (hydrogen donor) facilitates the conver-
sion of xylose to xylitol successfully without the utiliza-
tion of external hydrogen source for the hydrogenation
reaction. The synergistic effect of glycerol and Cu doping
on ZnO nanoparticles played a crucial role in the photo-
catalytic conversion of xylose. While catalyst increased
the yield up to a level, increase in temperature of reaction
from 30 °C to 50 2 °C decreased yield of xylitol, eryth-
ritol and DHA. The study showed the potential applica-
tion of photocatalytic conversion of biomass into valuable
products without the addition of acid/alkali, use of noble
metals, external hydrogen source, and high temperature or
pressure. Further studies are needed to improve process-
ing conditions so as to achieve higher yield.
Table 1 MS/MS fragments of
valuable compounds identifed
in the reaction product
Compound name
[M–H]−
Empirical formula
MS/MS fragment ion
DHA
Erythritol
Xylitol
89.03
121.04
151.05
C3H6O3
C4H10O4
C5H12O5
71.013, 59.013, 56.97, 41.02, 31.02, 29.03
103.04, 91.04, 89.03, 73.03, 61.03, 59.013, 43.0184
133.05, 119.034, 101.023, 89.023, 71.012, 59.012
1 3

Photocatalytic Conversion of Xylose to Xylitol over Copper Doped Zinc Oxide Catalyst
14. Granone LI, Sieland F, Zheng N et al (2018) Photocatalytic
Acknowledgements The authors thank the Director, CSIR-CFTRI
for his support. Authors thank Mr. Rajesh for glass fabrication of the
photocatalytic reactor set-up, Mr. Bavani Eswaran and Mr. Padmere
Mukund Laxman for their support for extended sample analysis facili-
ties. Authors also thank Dr. Basavarajappa H.T, Professor (Department
of Earth Science), University of Mysore for his support on Spectral
radiometer facility. Rohini B thanks UGC-Rajiv Gandhi National Fel-
lowship, New Delhi for her senior research fellowship.
conversion of biomass into valuable products: a meaningful
approach? Green Chem. https://doi.org/10.1039/c7gc03522e
15. Liu X, Duan X, Wei W et al (2019) Photocatalytic conversion
of lignocellulosic biomass to valuable products. Green Chem
21:4266–4289. https://doi.org/10.1039/c9gc01728c
16. Li S-H, Liu S, Colmenares JC, Xu Y-J (2016) A sustainable
approach for lignin valorization by heterogeneous photocataly-
sis. Green Chem 18:594–607. https://doi.org/10.1039/C5GC0
2109J
Compliance with Ethical Standards
17. Kenanakis G, Giannakoudakis Z, Vernardou D et al (2010)
Photocatalytic degradation of stearic acid by ZnO thin flms
and nanostructures deposited by diferent chemical routes. Catal
Today. https://doi.org/10.1016/j.cattod.2010.02.054
18. Kuriakose S, Satpati B, Mohapatra S (2014) Enhanced pho-
tocatalytic activity of Co doped ZnO nanodisks and nanorods
prepared by a facile wet chemical method. Phys Chem Chem
Phys. https://doi.org/10.1039/c4cp01315h
Conflict of interest The authors declare that there is no confict of in-
terest.
References
19. Akpan UG, Hameed BH (2010) The advancements in sol–
gel method of doped-TiO2 photocatalysts. Appl Catal A Gen
375:1–11
1. Shrotri A, Kobayashi H, Fukuoka A (2017) Catalytic conversion
of structural carbohydrates and lignin to chemicals, 1st edn.
Elsevier Inc., Japan
20. Kouklin N (2008) Cu-doped ZnO nanowires for efcient and
multispectral photodetection applications. Adv Mater. https://
doi.org/10.1002/adma.200701071
2. Azadi P, Inderwildi OR, Farnood R, King DA (2013) Liquid
fuels, hydrogen and chemicals from lignin: a critical review.
Renew Sustain Energy Rev 21:506–523
21. Yi G, Zhang Y (2012) One-pot selective conversion of hemicel-
lulose (Xylan) to xylitol under mild conditions. ChemSusChem
5:1383–1387. https://doi.org/10.1002/cssc.201200290
22. Vaiano V, Iervolino G, Rizzo L (2018) Cu-doped ZnO as ef-
cient photocatalyst for the oxidation of arsenite to arsenate
under visible light. Appl Catal B Environ 238:471–479. https
://doi.org/10.1016/j.apcatb.2018.07.026
3. Chen DH, Ye X, Li K (2005) Oxidation of PCE with a UV
LED photocatalytic reactor. Chem Eng Technol. https://doi.
org/10.1002/ceat.200407012
4. Fuente-Hernandez A, Corcos P-O, Beauchet R, Lavoie J-M
(2013) Biofuels and co-products out of hemicelluloses. In:
Fang Z (ed) Liquid, gaseous and solid biofuels—conversion
techniques, Intech publishing
23. Díaz-Álvarez A, Cadierno V (2013) Glycerol: a promising
green solvent and reducing agent for metal-catalyzed transfer
hydrogenation reactions and nanoparticles formation. Appl Sci
3:55–69. https://doi.org/10.3390/app3010055
5. Gallo JMR, Trapp MA (2017) The chemical conversion of bio-
mass-derived saccharides: an overview. a2—1,198. J Braz Chem
Soc 28:1586–1607. https://doi.org/10.21577/0103-5053.20170
009
24. Natarajan TS, Natarajan K, Bajaj HC, Tayade RJ (2011) Energy
efcient UV-LED source and TiO2 nanotube array-based reactor
for photocatalytic application. Ind Eng Chem Res. https://doi.
org/10.1021/ie200493k
6. Heikkilä H, Puuppo O, Tylli M, Nikander H, Nygrèn J, Lindroos
M, Eroma OP (1997) Method for producing xylitol. Patent
WO97/49659
7. Mikkola JP, Salmi T (2001) Three-phase catalytic hydrogena-
tion of xylose to xylitol—prolonging the catalyst activity by
means of on-line ultrasonic treatment. Catal Today. https://doi.
org/10.1016/S0920-5861(00)00530-7
25. Shie JL, Lee CH, Chiou CS et al (2008) Photodegradation
kinetics of formaldehyde using light sources of UVA, UVC
and UVLED in the presence of composed silver titanium oxide
photocatalyst. J Hazard Mater. https://doi.org/10.1016/j.jhazm
at.2007.11.043
8. Mikkola JP, Salmi T (1999) In-situ ultrasonic catalyst rejuve-
nation in three-phase hydrogenation of xylose. Chem Eng Sci.
https://doi.org/10.1016/S0009-2509(99)00058-5
26. Gupta A, Pandey A, Kumar A (2015) Wet chemical synthesis
and characterization of copper doped zinc oxide particle. J Chem
Pharm Res 7:532–537
9. Ghaznavi T, Neagoe C, Patience GS (2014) Partial oxidation
of d-xylose to maleic anhydride and acrylic acid over vanadyl
pyrophosphate. Biomass Bioenergy. https://doi.org/10.1016/j.
biombioe.2014.09.029
27. Rahmati A, Balouch Sirgani A, Molaei M, Karimipour M (2014)
Cu-doped ZnO nanoparticles synthesized by simple co-precipi-
tation route. Eur Phys J Plus. https://doi.org/10.1140/epjp/i2014
-14250-8
10. Hernandez-Mejia C, Gnanakumar ES, Olivos-Suarez A et al
(2016) Ru/TiO2-catalysed hydrogenation of xylose: the role of
the crystal structure of the support. Catal Sci Technol. https://
doi.org/10.1039/c5cy01005e
28. Sindhu R, Binod P, Pandey A (2016) Biological pretreatment
of lignocellulosic biomass—an overview. Bioresour Technol
199:76–82. https://doi.org/10.1016/j.biortech.2015.08.030
29. Ashokkumar M, Muthukumaran S (2014) Microstructure and
band gap tailoring of Zn0.96-xCu0.04CoxO (0 ≤ x ≤ 0.04) nano-
particles prepared by co-precipitation method. J Alloys Compd.
https://doi.org/10.1016/j.jallcom.2013.10.246
11. Xia H, Zhang L, Hu H et al (2020) Efcient hydrogenation of
xylose and hemicellulosic hydrolysate to xylitol over Ni–Re
bimetallic nanoparticle catalyst. Nanomaterials. https://doi.
org/10.3390/nano10010073
12. Zada B, Chen M, Chen C et al (2017) Recent advances in cata-
lytic production of sugar alcohols and their applications. Sci
China Chem 60:853–869
30. Sakai K, Kakeno T, Ikari T et al (2006) Defect centers and opti-
cal absorption edge of degenerated semiconductor ZnO thin
flms grown by a reactive plasma deposition by means of pie-
zoelectric photothermal spectroscopy. J Appl Phys. https://doi.
org/10.1063/1.2173040
13. Colmenares JC, Luque R (2014) Heterogeneous photocatalytic
nanomaterials: prospects and challenges in selective trans-
formations of biomass-derived compounds. Chem Soc Rev
43:765–778
31. Chen Q, Wang Y, Zheng M et al (2018) Nanostructures confned
self-assembled in biomimetic nanochannels for enhancing the
1 3

B. Rohini, H. U. Hebbar
sensitivity of biological molecules response. J Mater Sci Mater
Electron. https://doi.org/10.1007/s10854-018-0101-2
37. Estahbanati MRK, Feilizadeh M, Iliuta MC (2017) Photocatalytic
valorization of glycerol to hydrogen: optimization of operating
parameters by artifcial neural network. Appl Catal B Environ
209:483–492. https://doi.org/10.1016/j.apcatb.2017.03.016
38. Liu Q, Zhang T, Liao Y et al (2017) Production of C5/C6 sugar
alcohols by hydrolytic hydrogenation of raw lignocellulosic bio-
mass over Zr based solid acids combined with Ru/C. ACS Sus-
tain Chem Eng 5:5940–5950. https://doi.org/10.1021/acssuschem
eng.7b00702
32. Jiang T, Wang Y, Meng D et al (2014) Controllable fabrication of
CuO nanostructure by hydrothermal method and its properties.
Appl Surf Sci. https://doi.org/10.1016/j.apsusc.2014.05.116
33. Choi S, Do JY, Lee JH et al (2018) Optical properties of Cu-
incorporated ZnO (CuxZnyO) nanoparticles and their photo-
catalytic hydrogen production performances. Mater Chem Phys
205:206–209. https://doi.org/10.1016/j.matchemphys.2017.11.022
34. Labhane PK, Huse VR, Patle LB et al (2015) Synthesis of Cu
doped ZnO nanoparticles: crystallographic, optical, FTIR, mor-
phological and photocatalytic study. J Mater Sci Chem Eng
03:39–51. https://doi.org/10.4236/msce.2015.37005
39. Mishra DK, Dabbawala AA, Hwang JS (2013) Ruthenium nano-
particles supported on zeolite y as an efcient catalyst for selective
hydrogenation of xylose to xylitol. J Mol Catal A Chem 376:63–
70. https://doi.org/10.1016/j.molcata.2013.04.011
35. Kadam AN, Kim TG, Shin DS et al (2017) Morphological evolu-
tion of Cu doped ZnO for enhancement of photocatalytic activity.
J Alloys Compd. https://doi.org/10.1016/j.jallcom.2017.03.150
36. Payormhorm J, Chuangchote S, Kiatkittipong K et al (2017)
Xylitol and gluconic acid productions via photocatalytic-glu-
cose conversion using TiO2 fabricated by surfactant-assisted
techniques: efects of structural and textural properties. Mater
Chem Phys 196:29–36. https://doi.org/10.1016/j.matchemphy
s.2017.03.058
40. Liu C-J, Zhu N-N, Ma J-G, Cheng P (2019) Toward green produc-
tion of chewing gum and diet: complete hydrogenation of xylose
to xylitol over ruthenium composite catalysts under mild condi-
tions. Research 2019:1–9. https://doi.org/10.34133/2019/5178573
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Authors and Afliations
B. Rohini1,2 · H. Umesh Hebbar1,2
1
2
Academy of Scientifc & Innovative Research (AcSIR),
Food Engineering Department, CSIR – Central Food
Ghaziabad, Uttar Pradesh 201002, India
Technological Research Institute, Mysore, India
1 3