Soft template-based bismuth doped zinc oxide nanocomposites for photocatalytic depolymerization of lignin

Article abstract of DOI:10.1016/j.ica.2019.119390

Lignin depolymerization is a growing research area for the synthesis of aromatic molecules. In current studies, photocatalytic lignin depolymerization has been investigated by designing a series of soft template-based Zn_0.95Bi_0.05O nano-materials (SK1-SK5). Synthesized nano-materials were characterized by PXRD, SEM/EDX, BET, FTIR and utilized for photocatalytic lignin depolymerization in the presence of direct sunlight. Lignin depolymerization was monitored by UV–Visible Spectroscopy and catalytic pathway analyzed by different kinetic models. Percent depolymerization was studied for the effect of catalyst dose, starting lignin concentration and different temperatures. Pseudo second order was found the best fitted kinetic model indicating chemically rate controlled mechanism. Activation energy (E_a) for the reaction was found to be 11 kJ/mol that is remarkably lower than conventional lignin depolymerization i.e. 59.75 kJ/mol depicting better catalytic efficiencies of tested materials. Lignin depolymerization products were characterized by HPLC and FTIR Spectroscopy and their analysis indicated the conversion of lignin into smaller subunits like syringaldehyde, sinapyl alcohol, phthalates, and 4-hydroxy-benzoic acid. Specifically, phenol (21%) and 2-methoxy-4-methylphenol (16%) were detected by GCMS analysis in the products of lignin depolymerization by SK2.

Full text of DOI:10.1016/j.ica.2019.119390

Journal Pre-proofs
Research paper
Soft Template-Based Bismuth Doped Zinc Oxide Nano Composites for Photo-
catalytic Depolymerization of Lignin
Samia Kausar, Ataf Ali Altaf, Muhammad Hamayun, Muhammad Danish,
Muhammad Zubair, Sumbal Naz, Shabbir Muhammad, Muhammad Zaheer,
Shafiq Ullah, Amin Badshah
PII:
S0020-1693(19)31426-4
https://doi.org/10.1016/j.ica.2019.119390
ICA 119390
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
14 September 2019
26 November 2019
12 December 2019
Please cite this article as: S. Kausar, A. Ali Altaf, M. Hamayun, M. Danish, M. Zubair, S. Naz, S. Muhammad, M.
Zaheer, S. Ullah, A. Badshah, Soft Template-Based Bismuth Doped Zinc Oxide Nano Composites for Photocatalytic
Depolymerization of Lignin, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119390
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Soft Template-Based Bismuth Doped Zinc Oxide Nano Composites for
Photocatalytic Depolymerization of Lignin
Samia Kausara, Ataf Ali Altafa*, Muhammad Hamayuna, Muhammad Danisha, Muhammad
Zubaira, Sumbal Naza, Shabbir Muhammadb, Muhammad Zaheerc, Shafiq Ullahd, Amin Badshahd
aDepartment of Chemistry, University of Gujrat, Hafiz Hayat Campus, Gujrat 50700, Pakistan.
bDepartment of Physics, College of Science, King Khalid University, Abha 61413, P.O. Box 9004,
Saudi Arabia.
cDepartment of Chemistry and Chemical Engineering, SBA School of Science and Engineering,
Lahore University of Management Sciences (LUMS), Lahore, 54792, Pakistan.
dDepartment of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan.
*Corresponding Author: Ataf Ali Altaf, Department of Chemistry, University of Gujrat, Hafiz
Hayat Campus, Gujrat 50700, Pakistan. Email: atafali_altaf@yahoo.com
ABSTRACT
Lignin depolymerization is a growing research area for the synthesis of aromatic molecules. In
current studies, photocatalytic lignin depolymerization has been investigated by designing a series
of soft template-based Zn0.95Bi0.05O nano-materials (SK1-SK5). Synthesized nano-materials were
characterized by PXRD, SEM/EDX, BET, FTIR and utilized for photocatalytic lignin
depolymerization in the presence of direct sunlight. Lignin depolymerization was monitored by
UV-Visible Spectroscopy and catalytic pathway analyzed by different kinetic models. Percent
depolymerization was studied for the effect of catalyst dose, starting lignin concentration and

different temperatures. Pseudo second order was found the best fitted kinetic model indicating
chemically rate controlled mechanism. Activation energy (Ea) for the reaction was found to be
11kJ/mol that is remarkably lower than conventional lignin depolymerization i.e. 59.75 kJ/mol
depicting better catalytic efficiencies of tested materials. Lignin depolymerization products were
characterized by HPLC and FTIR Spectroscopy and their analysis indicated the conversion of
lignin into smaller subunits like syringaldehyde, sinapyl alcohol, phthalates, and 4-hydroxy-
benzoic acid. Specifically, phenol (21%) and 2-methoxy-4-methylphenol (16%) were detected by
GCMS analysis in the products of lignin depolymerization by SK2.
Key Words: Photocatalysis, Lignin Depolymerization, Bismuth Zinc oxide, Kinetics, Activation
Energy
1. Introduction
Conversion of lignin is a significant challenge owing to its high molecular weight polymeric nature
[1-3]. Lignin is resistant to microbial depolymerization acts as the main inhibitor of the natural
decay of plant materials. As it gives the structural support, preventing the contagious attack and
oxidative stress and also gives the impermeability [4]. Hence, depolymerization of lignin can lead
to the production of effective biofuels and several aromatic compounds like alcohols, ethers, other
synthetic polymers, vanillin, syringaldehyde, and different types of phthalates [5]. The lignin
depolymerization for the synthesis of such compounds has gained huge attention of the scientific
community during the last few decades.
Several chemical and biological pathways had been examined to convert lignin into smaller
derivatives or constituents, but they are not widely employed due to their high initial and operating
costs [6]. Photocatalytic depolymerization is found to be the most operative, environment-friendly

Commented [1]: Please do not use the word “effective” or
“efficient” or “effectiveness” three times in the same sentence. Try
to reply it by a sinonymous
and competent method due to its productivity, energy-saving, and cost-effectiveness [7, 8]. The
heterogeneous photocatalytic systems have been widely studied because of their ability to
photosensitize complete mineralization of a wide range of organic and polymeric substrates at
ambient temperatures and pressure [9, 10]. Also, this method is effective in depolymerization
without production of toxic by-products, including phenols, amides, and pesticides etc. [11-14].
Commented [SK2]: Revised as per recommendation
Metal doped materials are used well for catalytic depolymerization of lignin due to their efficient
conversion of macromolecules into smaller useful products as they provide extensive surface area
[15]. Yeber et al. [16] examined decolorization and reduction in acute toxicity of a cellulose
bleaching effluent by employing photocatalysis using ZnO based nanomaterials. Similar kind or
research showed that metal oxide-based catalysts could efficiently depolymerize the organic
compounds present in pulp and paper bleaching effluent [17].
Among all other transition metal photocatalysts, zinc oxide photocatalysts are found to be more
efficient in photocatalytic depolymerization and minimizing environmental problems. Also,
photocatalytic properties of ZnO can be enhanced by variating templates for depolymerization
processes [18]. Various kinds of soft templates have been employed to increase surface areas, pore-
volume, and stabilities of ZnO catalysts. Doped ZnO nanoparticles have been attracted much
attention due to their role in the enhancement of catalytic efficiency of materials and bismuth has
been reported as an important dopant material in ZnO [19, 20]. A wide range of applications of Bi
as dopant material can be studied due to a much bigger radius of Bi3+ (0.103 nm) than that of Zn2+
(0.072 nm). It produces large structural variation in the lattice hence variating its properties [21].
Commented [3]: Please revise the grammar of this sentence as
it is difficult to understand.
Commented [SK4]: Revised as per recommendation
Commented [5]: Plase revise the grammar in this sentence as
well.
Commented [SK6]: Revised as per recommendation
In the light of the importance of the subject, this study was aimed to synthesize a series of Bismuth
doped Zinc Oxide photocatalysts (SK1-SK5) by a new soft template sol-gel method for

depolymerization of lignin. Different factors affecting depolymerization i.e. catalytic dose, initial
lignin concentration, time and temperature along with their kinetics have been studied.
Commented [SK7]: Revised as per recommendation
2. Experimental
2.1 Chemicals and Reagents
Lignin was extracted from wood powder through literature reported organosolv acid treatment
method [22, 23]. Zinc acetate, bismuth acetate, ammonium hydroxide, sodium dodecyl sulfate
(SDS), cetyltrimethylammonium bromide (CTAB), dimethyl sulfoxide (DMSO), 1-butanol and
ethanol were purchased from Sigma Aldrich. All the chemicals were of analytical grade and used
without further purification.
Powder X-ray Diffraction (PXRD) measurements were performed in X-ray diffractometer
(Bruker, AXS D8) with Cu-Kα radiation (1.5406 Å) at scanning rate 10°min in the 2θ range from
10° to 90°. Morphological features of catalysts were characterized by Scanning Electron
Microscope (SEM) (JEOL, JSM-6360 EO) and elemental composition was estimated by Energy
Dispersive X-Ray (EDX) Diffractometer (JEOL JSM-6360 LV). The surface area and pore
structure parameters were calculated by Brunauer-Emmett-Teller (BET) analysis performed
through Quantachrome Nova 2200e. FT-IR spectra were recorded on Bruker Alpha Platinum ATR
Spectrophotometer using KBr pellets in the 4000-400 cm-1 range. Catalytic activity monitoring
was done through High Performance Liquid Chromatography (HPLC) (Shimadzu, CBM-20A,
UV-Vis. Detector SPD 20A, column: C-18 (250 x 4.6 mm), pump: LC 20AT, Mobile Phase:
Acetonitrile, λ=295nm, injection volume: 1.5µL and temperature: 35ºC). Catalytic
depolymerization was studied by Ultraviolet (UV) Spectrophotometer (Shimadzu) in the

frequency range of 250–800 nm. Solid Product of Lignin depolymerization was Characterized by
Gas Chromatography Mass Spectrometry (GCMS) (Shimadzu QP2010, MS Detector SPD 20A).
2.2 Synthesis of bismuth doped zinc oxide Photocatalysts (SK1-SK5)
Zn0.95Bi0.05O in water (SK1): Sample SK1 was prepared by sol-gel method. 0.915g of zinc acetate
and 0.058g of bismuth acetate were mixed in 100mL of water and stirred well for half an hour.
5mL of 25% ammonium hydroxide solution was added drop-wise to the reaction mixture with
constant stirring. The gel was formed in the container and filtered off. The Gel obtained was dried
at room temperature and grinded well. The dried powder was calcinated at 400˚C in the muffle
furnace for 24 hours. The material (SK1) obtained as the white powder was characterized and
utilized for catalytic studies. PXRD;D (42.45±7); Elemental percentage composition (by EDX)
calculated for Zn0.95Bi0.05O is Bi 11.80; Zn 70.14; O 18.06 found Bi 11.90; Zn 70.09; O 18.01;
FTIR cm-1 ʋ(Zn-O) (1628, 642)
Commented [8]: These quantities are based in any previous
study or why have you selected them? Please explain it and do it as
well in the following descriptions.
Commented [SK9]: All rationales have been discussed in
introduction. We have used these quantities in accordance to
literature survey [ref. [16, 18-21], In literature such ratios are found
for the decomposition of organic pollutants with good efficiency.
Zn0.95Bi0.05O in ethanol (SK2): SK2 was synthesized in the same as for SK1 by utilizing the 100
mL ethanol in place water in the first step. The material (SK1) obtained as white powder was
characterized and utilized for catalytic studies. PXRD; D (36.42±12); Elemental percentage
composition (by EDX) calculated for Zn0.95Bi0.05O is Bi 11.80; Zn 70.14; O 18.06 found Bi 11.65;
Zn 70.10; O 18.25; FTIR cm-1 ʋ(Zn-O) (1626 and 637)
Commented [SK10]: Revised as per recommendation
Zn0.95Bi0.05O in water/butanol and DMSO (SK3): Sample SK3 was prepared by template-assisted
sol-gel method. 0.915g of zinc acetate and 0.058g of bismuth acetate were mixed in 100mL of
water and stirred well for half an hour. 25mL n-butanol was added to the solution and stirred well.
15 mL DMSO was added to the mixture to break the layers of water and 1-butanol, 5mL of 25%
ammonium hydroxide solution was added drop-wise to the reaction mixture with constant stirring.

The gel was formed in the container and filtered off. The Gel obtained was dried at room
temperature and grinded well. The dried powder was calcinated at 400˚C in the muffle furnace for
24 hours. The material (SK3) obtained as white powder was characterized and utilized for catalytic
studies. PXRD; D (35.73±11); Elemental percentage composition (by EDX) calculated for
Zn0.95Bi0.05O is Bi 11.80; Zn 70.14; O 18.06 found Bi 11.71; Zn 70.21; O 18.08; FTIR cm-1 ʋ(Zn-
O) (1635 and 637).
Zn0.95Bi0.05O in water/butanol and SDS (SK4): SK4 was synthesized in the same as for SK3 by
utilizing the 3.6g SDS in place of 15 mL DMSO. The material (SK4) obtained as white powder
was characterized and utilized for catalytic studies. PXRD; D (32.85±8); Elemental percentage
composition (by EDX) calculated for Zn0.95Bi0.05O is Bi 11.80; Zn 70.14; O 18.06 found Bi 11.88;
Zn 70.08; O 18.04; FTIR cm-1 ʋ(Zn-O) (633 and 606).
Zn0.95Bi0.05O in water/butanol and CTAB (SK5): SK5 was synthesized in the same as for SK3 by
utilizing the 5.9g CTAB in place of 15 mL DMSO. The material (SK5) obtained as white powder
was characterized and utilized for catalytic studies. PXRD; D (34.13±7); Elemental percentage
composition (by EDX) calculated for Zn0.95Bi0.05O is Bi 11.80; Zn 70.14; O 18.06 found Bi 11.93;
Zn 70.19; O 17.88; FTIR cm-1 ʋ(Zn-O) (629).
2.3 Extraction of Lignin
Extraction of lignin from the wood powder of Oryza sativa was carried out using a mixture of
formic acid/acetic acid/water. This organosolv treatment under acidic conditions cleaves ether
bonds between lignin and hemicellulose, thereby accelerating the delignification process [23-25].
10g of wood powder (Oryza sativa) soaked into 15 mL water in the round bottom flask. 85 mL
mixture of formic acid and acetic acid (7:3) were added to the soaked wood powder. The reaction
Commented [11]: The grammar of this sentence is not
acceptable, please re-do it.
Commented [SK12]: Revised as per recommendation
Commented [SK13]: Revised as per recommendation

mixture was heated at 80°C with constant stirring for two hours. After this pulping process, the
reaction mixture was filtered and washed with 80% formic acid and distilled water. Then, 80ml of
35% H2O2 solution, 80ml of 85% mixture of formic acid and acetic acid (7:3) are added to pulp
obtained. Mixture heated at 80°C with constant stirring for two hours, filtered after cooling and
washed with hot distilled water.
Liquor obtained was heated for 2 hours at 105˚C. It was concentrated by rotary evaporation to
reduce the volume of about 40mL. 200mL distilled cold water was added to the concentrated
liquor. Upon the addition of water, lignin was precipitated in the form of brown powder. The lignin
obtained in this way was characterized by FTIR spectroscopy and HPLC.
2.4 Photocatalytic Activity
Photocatalytic lignin depolymerization activity was done according to literature reported method
[26, 27] with some modifications. To perform photocatalytic activity, 100ppm lignin (L1) solution
in dioxane was prepared. 50mL of L1 solution was subjected to stirring under sun light in the
presence of 0.1g catalysts (SK1). Aliquots of 5mL from reaction mixture were taken at different
time intervals of 0, 5, 10, 15, 30, 60 and 90-minutes. UV spectrum was taken from UV
spectrophotometer for all the taken aliquots at different time intervals. From the spectrum,
absorbance at 285nm was noted and %depolymerization of lignin was calculated for all the sample
aliquots by following Eq. 1 [26].
Commented [14]: This is a very low concentration of lignin.
Have you tried other concentrations? I think that it would be
necessary to include experiments at higher concentrations to
evaluate the depolymerization capacity.
Furthermore, I don’t find any experiment done here about the
influence of lignin concentration . The only data regarding lignin
concentration that I have found is 50mL of 100ppm lignin solution .
Are there more?
Commented [SK15]: We have revised the photocatalytic
activity procedure in section 2.4 to make it clear as per your
recommendation. Influence for different lignin concentrations (50,
100, 150 and 200ppm) elaborated in section 4.1.2.
퐶표 ― 퐶푡
------ (1)
% 퐷푒푝표푙푦푚푒푟푖푧푎푡푖표푛 =
× 100
퐶표
Where, C0 is the initial concentration and Ct is the concentration at different sampling times. The
concentration of lignin after reaction time was measured by calibration curve (e.g. Fig. S1a in
supplemental data) between the absorbance and known lignin concentration. Unknown lignin

concentration for measured absorbance at different time intervals calculated by linear equation of
known data. Same method was adopted to perform catalytic activity for all other catalysts (SK2-
SK5).
Kinetic parameters were calculated by variating i.e. initial lignin concentration (50, 100, 150 and
200ppm) and catalyst dose variation (0.04, 0.06, 0.08 and 0.1g) at different time intervals by
following the same procedure as described above. The thermodynamic parameters were calculated
by repeating the same procedure at various temperatures i.e (25, 30, 45, 60°C). Representative
samples (L1-SK2, L1-SK3) after photocatalytic lignin depolymerization activity were analyzed
by HPLC. Solid product (L1-SK2) obtained, after the evaporation of dioxane, from lignin
depolymerization activity was dissolved in the formic acid and analyzed by GCMS.
3. Results and Discussion
3.1 Characterization of SK1 to SK5
Powder X-Ray Diffraction (PXRD) and FTIR Analysis:The phase composition and particle size of
the synthesized materials of all the series were characterized by PXRD and data obtained is
reported in Table 1. PXRD patterns of SK1-SK5 (presented in Fig. 1) matched with standard
PXRD patterns of ZnO (JCPDS File Card No.05-0664, 36-1451) and Bi2O3 (JCPDS: 712274 &
501088) [28, 29]. All the diffraction peaks reflect samples indexed to the hexagonal wurtzite
structure of ZnO in accordance with standard data of ZnO hexagonal crystalline system (JCPDS
card No. 36-1451) [30]. In ZnBi2O4 the (101) diffraction peak is predominant to overall other
diffraction peaks in the sample SK5. The peaks marked ‘■’ were attributed to Bi2O3 (JCPDS:
712274 & 501088). This trend suggested ZnO as major phase of synthesized materials (SK1-SK5)
whereas minor phase represented Bi2O3. Much less intensity of Bi2O3 showed its presence as minor

phase. The presence of ZnBi2O4 attributed to the immigration of Zn atoms into the interstitial
positions of the regular zinc sites caused by the substitution of Bi. This peak assignments in PXRD
jointly with EDX results indicate that sample SK1-SK5 are composite materials with the elemental
composition of ZnBi0.015O having crystalline phases of ZnO, Bi2O3 and ZnBi2O4.
Figure 1: PXRD Pattern of Nanomaterials (SK1-SK5). [Diffraction Peaks of ZnO Agree well with
Standard Pattern (JCPDS NO. 05-0664, 36-1451) while Diffraction Peaks of Bi2O3 corresponds
with Standard (JCPDS NO. 00-027—0053, 71-2274, 78-1793, 41-1449) and diffraction peaks of
ZnBi2O4 in accordance with (JCPDS NO. 41-0253)]
Commented [16]: different font sizes have been used here.
Please correct it. In the figure, please substitute the word “theta”
by the symbol.
Commented [SK17]: Revised as per recommendation
After PXRD phase assignments, other parameters i.e. crystallite size, dislocation densities,
crystallite volume, and microstrain were calculated for all the synthesized materials and reported
in Table 1. Particle size calculation is performed using the Debye Sherrer formula [31] as follows:
0.91λ
D=
----- (2a)
훽푐표푠θ

Where D is the particle size of the sample and λ is the wavelength of the X-ray diffraction beam
and has a value of 1.5405980. The angle at which diffraction occurs is cos θ, and β is the FWHM
(full width at half maximum) value [32].
The average crystallite size was calculated at their corresponding intensities was 42.45nm for SK1,
36.42nm for SK2, 35.73nm for SK3, 32.85nm for SK4 and 34.13nm for SK5. SK3 showed the
intensity and stability of peaks enhanced due to the presence of SDS surfactant which enhances
the particle size of the catalyst. Dislocation density (δ), which represents the extent of defects in
the sample is defined as the length of dislocation lines per unit volume of the crystal and is
calculated using the Eq. 2b [33].
ퟏ
-------- (2b)
휹 =
푫ퟐ
The extent of defect found to be less in photocatalysts having smaller crystallite sizes showing
greater stability. The volume of crystalline (V) is calculated using the relation (V= D3) [34] which
indicates that there is proper incorporation of Bi ions into regular lattice sites of O-. The strain-
induced broadening in powders due to crystal imperfection and distortion is calculated as
microstrain (ε) [30]. Positive values of ε represent tensile strain while a negative value represents
compressive strain. Micro strain for nanoparticles was calculated by Eq. 2c [30] as follows:
휷
------- (2c)
휺 =
ퟒ풕풂풏휽
The inclusion of Bi3+ in the ZnO structure causes an increase in the strain and increasing crystal
imperfection [29-32]. Positive values depicted tensile microstrain in SK1-SK5 catalysts which can
be due to the lattice shrinkage observed in the calculation of lattice parameters [35]. All
synthesized materials (SK1 to SK5) were analyzed by FTIR spectroscopy to predict different metal
oxygen bonds stretching vibrations and results obtained have been presented in Fig. S2(a)

(supplemental data). Generally metal oxides show stretching bands in the region below 1000cm-1
due to inter-atomic vibrations [36]. In our materials these signals appeared in the characteristic
region of the spectra, as listed in Table 3, in accordance with literature [37].
Table 1: Synthetic Conditions, Composition and PXRD Parameters of Nanomaterials (SK1-SK5)
Found
Average
Dislocation Density × Micro
Synthetic
Conditions
Volume
V=D3
Samples
SK1
Composition Crystallite Size
(by EDX)
10-3 (nm)-2
Strain
D (nm)
42.45±7
(δ)
(ε)
0.915g
Zinc
Acetate, 0.058g
Bismuth Acetate,
NH4OH, Water
0.915g
Acetate,
Bismuth Acetate,
NH4OH, Ethanol
0.915g
Acetate, 0.058g
Bismuth Acetate,
NH4OH,
DMSO/Butanol in
Water
Zn0.95Bi0.05
O
O
82087
62729
5.98×10-7
7.54×10-7
0.022
0.031
SK2
SK3
SK4
Zinc
0.058
Zn0.95Bi0.05
36.42±12
35.73±11
Zinc
Zn0.95Bi0.05
O
O
O
59342
41250
44612
7.83×10-7
9.27×10-7
8.58×10-7
0.028
0.032
0.029
0.915g
Zinc
Acetate, 0.058g
Bismuth Acetate,
NH4OH,
SDS/Butanol in
Water
Zn0.95Bi0.05
32.85±8
34.13±7
0.915g
Zinc
SK5
Acetate, 0.058g
Bismuth Acetate,
NH4OH,
Zn0.95Bi0.05
CTAB/Butanol in
Water
Scanning electron microscopy (SEM) and EDX Analysis: SEM and EDX parameters calculated
for nanomaterials (SK1-SK5) are given in Table 2(a). SEM images of ZnO doped with bismuth
for SK5 are shown in Fig. 2(A) and (B). Images present heterogeneous morphology incorporating

elliptical leaf-like structures. While SEM images of SK4 are shown in Fig. 2(C) and (D) which
represents the nanograins like texture showing agglomerated spherical particles type surface
morphology with rugged appearance.
The high-intensity peaks of Zn Lα peak at about 1.0 and 8.8keV for SK4 while peaks at about 1.5
and 10.5keV for SK5 indicates the ZnO presence with major elemental composition with the
percentage about 60.38%. Bi Kα peaks at 1.5-3.0KeV with minor representation and O Kα peak
can also be observed in the spectra presented in Fig. S1b (supplemental data). This elemental
composition from EDX is in accordance with PXRD results which are indicating the ZnO as a
major phase while minor represented by Bi2O3.
Table 2(a): SEM and EDX Parameters for Synthesized Nanomaterials (SK1-SK5)
Samples
Properties
SK1
SK2
SK3
SK4
SK5
Expected Unit
Formula
ZnBi0.002
O
ZnBi0.002
O
ZnBi0.002
O
ZnBi0.002
O
ZnBi0.002O
Found Unit
Formula
Zn0.95Bi0.05
O
Zn0.95Bi0.05
O
Zn0.95Bi0.05
O
Zn0.95Bi0.05
O
Zn0.95Bi0.05O
Found
Bi (11.80); Zn Bi (11.65); Zn Bi (11.71); Zn Bi (11.88); Zn Bi (11.93); Zn
Percentage
Composition
Color/Nature
(70.14); O
(18.06)
(70.10); O
(18.25)
(70.21); O
(18.08)
(70.08); O
(18.04)
(70.19); O
(17.88)
White Powder White Powder White Powder White Powder White Powder
Composite Composite Composite Composite
Material Nature Composite
Dispersity
Structural
Appearance
Heterogeneous Heterogeneous Heterogeneous Heterogeneous Heterogeneous
Elliptical Leaf Agglomerated Elliptical Leaf Elliptical Leaf Agglomerated
Like structures Spherical
Particles
Like structures Like
Structures
Spherical
Particles

Figure 2: SEM Images of SK5 [A(5µm) and B(1µm)] and of SK4 [C(5µm) and D(1µm)]
Brunauer-Emmett-Teller (BET) Analysis: The specific surface area of nanomaterials (SK1-SK5)
calculated by multipoint BET and pore structure parameters were calculated from the DFT method
are enlisted in Table 2(b). The specific surface area of nanomaterials was in the range of 8-25
m2/g. SK4 has the maximum surface area (25.53 m2/g) but lesser pore volume (0.009) indicated
material was microporous in nature. Whereas, SK2 and SK3 have 18.45 and 17.30m2/g specific
surface areas and 0.029 and 0.039 pore volumes respectively indicating the mesoporous nature of
materials. Values of constant C for SK1-SK4 were <100 which indicated strong adsorbate-
adsorbent interaction. For SK5 value was 272.82 which is due to the high porosity of the material.
Comparison of experimental data with BET results exhibited that varying the soft templates for

synthesis of nanomaterials, significantly affected the specific surface area and pore size
distribution of nanomaterials.
Table 2(b): BET Properties of Synthesized Nanomaterials (SK1-SK5)
Samples
BET Properties
SK1
12.002
0.032
4.093
37.670
SK2
18.45
0.029
4.093
172.75
SK3
17.302
0.039
4.093
22.015
SK4
25.530
0.048
4.093
28.726
SK5
7.539
0.009
3.537
272.827
Surface Area (SBET) (m2/g)
Pore Volume (Vm) (cc/g)
Pore Width (nm)
Constant C
Pore Structure Parameters
Cumulative Surface Area of
Pores (SDFT) (m2/g)
Cumulative Pore Volume
(VDFT) (cc/g)
11.2
0.0148
7.25
10.17
0.013
7.05
12.59
0.017
7.15
17.48
0.023
7.25
4.56
0.0049
7.23
Cumulative Pore width (nm)
3.2 Characterization of Extracted Lignin (L1)
For lignin extraction organosolv acid treatment allows an effective dissolution of hemicellulose
content contained in the wood powder by the breakdown of ether linkages between lignin and
hemicellulose, liberating lignin [23]. Hence lignin obtained through this method is mostly of high
purity and devoid of any cellulosic and sugar contents [38].
FTIR spectrum of extracted lignin is compared by literature reported high purity lignins as shown
in Fig. S2(b) in supplemental data. The FTIR spectra have shown some characteristic absorption
peaks which indicates the presence of some structures and functional groups specific to lignin.
Weak signal appeared at 3367cm-1 is due to the associated OH group. The signal appeared at
1714cm-1 due to the presence of unconjugated ketones, carbonyls, and ester groups and signals at
-1
1597, 1512, 1454 and at 1424, 1370cm depicted the aromatic benzene ring functionalities and
stretching vibrations for methyl groups. Signal appeared at 1326cm-1 because of Syringyl ring
depicting stretching vibrations of ʋ(C-O) group whereas signals at 1270-1275cm-1 indicate the ʋ(C-

O) vibration for Guaiacyl ring. Absorption band appeared at 1028cm-1 due to the presence of
aromatic ring and primary alcohols, these structures showing the stretching vibrations of ʋ(C-O,
C-H) groups [39]. FTIR vibrations of all lignin components [40-42] are listed in Table 3.
Commented [18]: There are a lot of sentences that are not
properly expressed in English. Please make a revision of the whole
document in terms of grammar.
Commented [SK19]: Revised as per recommendation
Table 3: IR Vibrations of Extracted Sample of Lignin (L1)
Stretching Vibrations
Structures Present in Lignin
Wavenumbers/cm-1
ʋ(O-H)str
ʋ(C=O)str
Hydroxyl group
3367 cm -1
1714 cm -1
ketones,
Unconjugated
Carbonyls, and ester groups
Benzene ring
ʋ(aromatic)str
ʋ(C-H)str
1597, 1512, 1424 cm -1
1454, 1370 cm -1
1326 cm -1
Methyl group
ʋ(C-O)str
Syringyl ring
ʋ(C-O)str
Guaiacyl ring
1424 cm -1
1028 cm -1
ʋ(C-O,C-H)str
Aromatic ring and primary
alcohol
ʋ(Zn-O)
1628 and 642cm-1 (SK1), 1626 and
637cm-1 (SK2), 1635 and 637 cm-1 (SK3),
633 and 606 cm-1 (SK4), 629 cm-1 (SK5)
HPLC analysis of lignin has shown different peaks as shown in Fig. S7 (supplemental data) which
indicated the affinity of the lignin sample with the stationary phase. Hence, elution time
characterized the sample to be high molecular weight. High-intensity peaks indicating major
component present at a high concentration which decrease by the depolymerization action of
catalysts. Different parameters from HPLC of lignin are enlisted in Table 4. Response factor [43]
was calculated for known lignin concentration injected as follows:
Commented [20]: Could you please include information about
the used standards to make the analysis? They could deeply
influence the interpretation of your results. Furthermore, as I have
mentioned before, some sugars and hemicellulosic-derived
products may appear within the lignin in the sample and should be
determined here and indicated in order to define the quality and
purity of the lignin sample.
Commented [SK21]: HPLC is only used to compare elution
trend of lignin with its depolymerized products in accordance with
literature reported characterization [ref. 22].
푃푒푎푘 퐴푟푒푎
------- (3)
푅푒푠푝표푛푠푒 퐹푎푐푡표푟 (푅퐹) =
퐶표푛푐푒푛푡푟푎푡푖표푛 표푓 퐿푖푔푛푖푛

Table 4: HPLC Data of 100µg/mL Lignin before Photocatalytic Activity
Peak No HPLC Data of Lignin (100µg/mL) before Photocatalytic Activity
Retention Time
Intensity
Peak Area
Response Factor
(RF)
(min)
4.7
1
2
3
4
5
6
1118
977
5254
4396
5011
29062
1600
3554
52.54
4.5
43.96
3.9
1285
9375
1143
3231
50.11
3.1
290.62
16
1.4
1.1
35.54
4. Catalytic Activity Studies
4.1 Photocatalytic Depolymerization of Lignin by Nanocomposites (SK1-SK5)
Commented [22]: Which lignin sample, 50mL of 100ppm lignin
solution ??
Absorbance at 285nm measured for all the aliquots taken during photocatalytic lignin (L1)
depolymerization activity at different time intervals by UV spectrophotometer. From the
absorbance, %depolymerization of lignin was calculated as described in section 2.4.Abs285 can be
assigned to the aromatic ring, and the decrease in it is closely associated with the transformation
of the aromatic ring [44]. The peaks at 285nm tend to decrease with the increase of photocatalytic
reaction time indicating the decomposition of the aromatic ring. Similarly, different factors like
the effect of catalytic dose, initial lignin concentrations and temperature were investigated at
different time intervals by calculating %depolymerization of lignin. Nanocomposites showed
higher photocatalytic activity with an increase in percent lignin depolymerization at different time
intervals (Fig. 3) as depicted in Table 5.
Commented [SK23]: We have revised and discussed in detail
in the section 2.4 to make it clear.
Maximum 45% of depolymerization observed in presence of SK1 whereas SK4 and SK5 have
shown 12% and 15% conversion as shown in Fig S3(a), (d)-(e)) in supplemental data. Among all

the catalysts, SK2 and SK3 have shown efficient catalytic activity to depolymerize the lignin. For
100mg/L of SK2 used for depolymerization of 100ppm lignin, after 15min of irradiation under
sunlight decrease in absorption intensity observed. 75% depolymerization was found after 15min
of irradiation which was increased to 82%, 90% and 97% after 30, 45 and 60min for SK2
[supplemental data (Fig. S3(b)] 74% depolymerization was found after 15 min of irradiation which
was increased to 80%, 88% and 96% after 30, 45 and 60min for SK3 [supplemental data (Fig.
S3(c)]. The maximum catalytic efficiency of SK2 and SK3 to depolymerize lignin attributed to
their larger specific surface areas calculated from BET analysis (Table 2(b)) which provides the
maximum number of active sites. The less catalytic efficiency of SK5 besides having the greater
surface area as compared to others is may be due to its microporous nature and lesser adsorbate-
adsorbent interactions.
Table 5: Photocatalytic Depolymerization of Lignin (L1100ppm) by 0.1g SK1-SK5 at Different
Time Intervals
Time (min) Percentage Depolymerization of Lignin (L1) with 0.1g of SK1-SK5
SK1
SK2
45
SK3
43
SK4
SK5
5
9
1
1
10
15
30
45
60
55
52
12.5
15.7
19
2
1.8
6.5
10
75
74
6
82
80
7.8
10.3
12
90
88
23
12.5
15
97
96
47

Figure 3: %Depolymerization of Lignin100ppm by Photocatalysts SK1-SK5 (0.1g) at different Time
Intervals
4.1.1 Effect of Catalytic Dose
The effect of catalyst concentration on the depolymerization of lignin was examined by changing
the ZnO catalyst dosage from 0.04 to 0.1g/L for 60 min irradiation of sunlight. Depolymerization
percent of lignin was calculated for SK2 and SK3 (100ppm of L1) at different time intervals as
shown in Table 6. %depolymerization found to be increased with increasing catalyst dose from
0.04g/L to 0.1g/L for SK2 (Fig. S4(a) and SK3 (Fig. S4(b) in supplemental data.
The highest lignin depolymerization (97%) was achieved at 60 min irradiation for dosage 0.1g/L
of ZnO. The maximum increase in the dosage of ZnO (0.1g/L) results in increased lignin
depolymerization possibly contributed by increased surface provided by catalyst resulting in not
only more photons arriving at the surface of catalysts but also the increase in the passage of
irradiation through the lignin solution. Correlation between %depolymerization of lignin and
catalyst dose was found to be significant as R2 value calculated to be around 0.99 (Fig. 4).

Table 6: %Depolymerization of Lignin by Different Catalytic Doses of SK2 and SK3 at Different
Time Intervals
Time (min) Percentage Depolymerization of Lignin (L1) with different Catalytic Dose
0.04g
SK2
0
0.06g
SK2
0
0.08g
SK2
0
0.1g
SK2
0
SK3
0
SK3
0
SK3
0
SK3
0
0
15
30
45
60
2
1
21.5
24
32
40
53
63
32
48
56
68
78
75
74
80
88
96
18
29
35
41
32
82
21
31
39
90
21
35
42
97
Figure 4: Correlation between %Depolymerization of Lignin by Different Catalytic Doses (0.04-
0.1g) of SK2 and SK3 at Different Time Intervals
4.1.2 Effect of Lignin Initial Concentration

The effect of initial lignin concentration on lignin depolymerization was evaluated with lignin
concentration varied from 50 to 200ppm and monitoring the %depolymerization of lignin (Table
7). Photocatalytic depolymerization of lignin by ZnO photocatalysts was found to decrease with
the increase in lignin concentration, especially when lignin concentration >100 mg/L as shown in
Fig. S5 in supplemental data.
The decreased efficiency under higher initial lignin concentration is attributed to the adsorption of
more lignin molecules on the surface of the catalyst. This results in a large decrease in a number
of active sites on the catalyst surface and thus less generation of active species. Also, an increase
in lignin concentration causes a decrease in the number of photons reaching the surface of catalysts,
resulting from less sunlight available for the exciton of catalyst particles due to more possible
absorption by lignin molecules. However, 97% of lignin depolymerization was achieved after 60
min initial lignin concentration was 100ppm. Correlation between %depolymerization of different
lignin concentrations by SK2 at different time intervals was found to be around 0.9 depicted in
Fig. 5.
Commented [24]: The obtained results are not very congruent
as you indicate that the depolymerization percentage decreases
with lignin concentration (as could be expected) but you don’t
explain why have you obtained higher depolymerization
percentages with 150 ppm than with 200 ppm or why the best
results are presented with 100 ppm. I think that this discussion is
the most important part of the work and should be much deeply
done. The influence of the lignin concentration in the
depolymerization rate is the fact that would allow to use this
method or not for a further obtaining of added-value components
from lignin.
Table 7: %Depolymerization of Different Lignin Concentrations by SK2 at Different Time
Intervals
Time
(min)
0
%Depolymerization of Lignin (L1) at Different Lignin Concentrations
Commented [SK25]: Trends for lignin depolymerization on
increasing lignin depolymerization are as expected and elaborated
in second paragraph of section 4.1.2.
But at the moment we are unable to understand the
depolymerization behavior at 50ppm lignin concentration.
50ppm
0
100ppm
150ppm
200ppm
0
0
0
15
40
75
82
90
97
8
25
30
58
11
15
20
30.15
50.25
58.3
45
60
60
64

Figure 5: Correlation between %Depolymerization of Different Lignin Concentrations (50-
200ppm) by SK2 at Different Time Intervals
4.1.3 Effect of Temperature
The temperature dependence of the depolymerization rate constant of lignin by 0.1g SK2 (using
100ppm of L1) was investigated at 25, 35 and 55 and 65°C. %depolymerization of lignin was
calculated at different time intervals at each temperature. The rate of depolymerization reaction tends
to increase with increasing temperature as shown in Table 6. Maximum %depolymerization obtained
at 25°C with 0.1g SK2 which showed a decrease in %depolymerization with increasing temperature
as shown in Fig. S6 (supplemental data). The decrease in %depolymerization of lignin in the tested
temperature range can be explained by an increase in the thermal stability due to the increasing
aromatic character of the lignin-derived char. High temperatures turn lignin into a highly crosslinked
carbonaceous material [45]. Correlation between %depolymerization of lignin by SK2 at different
temperatures was found to be around 0.9 depicted in Fig. 6.

Table 8: %Depolymerization of Lignin (100ppm) by SK2 at Different Temperatures and Time
Intervals
Time
(min)
0
%Depolymerization of 100ppm Lignin (L1) at Different Temperatures
25⁰C
0
35⁰C
0
55⁰C
0
65⁰C
0
15
75
27
18
13
30
45
60
82
90
99
54
60.6
72
23
36.5
47.8
14.3
23.6
35
Figure 6: Correlation Between %Depolymerization of Lignin (100ppm) by SK2 at Different
Temperatures
4.2 Kinetics Study of Lignin Depolymerization
Pseudo kinetics models such as; Pseudo first-order kinetics and pseudo second-order are more
frequently used to explain the adsorption kinetics of solid/liquid systems [46-48]. Kinetics of lignin
depolymerization on the surface of ZnO was illustrated best by pseudo second-order kinetic model.
The equation of pseudo second-order reaction is as follows:

푡
1
푡
------- (3)
=
+
푘2퐶2푡
퐶푡
퐶푒
For all the factors studied, the chemical reaction seems significant in the rate-controlling step. The
values of correlation constant R2 at 0.04g, 0.04g, 0.04g, and 0.04g catalyst dose were close to unity
as shown in Fig 7. Whereas, values of correlation constant R2 for 50, 100, 150 and 200 ppm initial
lignin concentration was close to unity depicted in Fig 8. Pseudo-second order chemical reaction
kinetics provide the best correlation of the experimental data and the reaction mechanism is found
to be chemically rate-controlling [46].
Figure 7: Pseudo Second Order Kinetics of Lignin Depolymerization by different Catalytic
Doses (0.04-0.1g) of SK2 and SK3

Figure 8: Pseudo Second Order Kinetics of Lignin Depolymerization by different Initial Lignin
(L1) Concentrations (50-200ppm)
4.2.1 Activation Energy (Ea)
The depolymerization stage in temperature treatment is kinetically controlled by an energy barrier
that comprises multiple processes; i.e., breaking hydrogen bonds, water evaporation, or water
diffusion among others [49]. Hence Ea values dependency on conversion may provide useful
mechanistic clues such as the number of presumable reaction steps involved in depolymerization of
lignin [50]. Pseudo second-order kinetics was applied to the data obtained by temperature variation
on photo depolymerization of lignin depicted in Fig 9. From which we can see that the apparent
depolymerization rate kinetic constant (k) correspondingly increased with increasing temperature as
shown in Table 9.
Table 9: Pseudo Second Order Kinetic Parameters at Different Temperatures
Pseudo Second Order Kinetic Parameters at Different Temperatures
Temperature
Reaction Rate (k)
Ce
R2

25°C
50°C
75°C
100°C
0.25
0.31
0.41
0.43
0.0006
0.0019
0.0051
0.0088
0.971
0.957
0.974
0.980
Figure 9: Pseudo Second Order Kinetics of Lignin Depolymerization at different Temperatures
Linear curves were obtained shown in Fig. 10. by plotting the logarithm of apparent
depolymerization kinetic constants (lnk) against 1/T in the solutions approaching 0.93, which was
mainly following Arrhenius equation (k = A−Ea/RT) rearrangement [51] as follows
푙푛푘 = ― 푅퐸푇 +푙푛퐴
푎
--------(4)
Where k is the reaction constant, A is the frequency factor (h−1), Ea is the activation energy
(kJ/mol), R is the gas constant (8.314 J/mol·K), and T is the temperature (K)). The activation
energy (Ea) for depolymerization calculated from the slope of the line (Slope = -Ea/R) was

calculated to be 11kJ/mol. According to the rule, higher Ea indicates less depolymerization
reaction was influenced by temperature. The calculated activation energy was remarkably lower
than that of conventional depolymerization of lignin i.e. 59.75 kJ/mol [52]. Hence lower activation
energy value indicated the efficiency of catalyst to depolymerize lignin more conveniently.
-0.8
-1
-1.2
-1.4
-1.6
2.9
3
3.1
3.2
3.3
3.4
1/T
Figure 10: Arrhenius Law on Temperature Data
5. Characterization of Depolymerization Products
FTIR Analysis: The FTIR spectrum of lignin before and after depolymerization through catalytic
activity presented in Fig. 11 showed characteristic absorption frequencies indicating the presence
of different functionalities matched with degradation products of lignin i.e. syringaldehyde, 4-
hydroxy-benzoic acid phthalates and sinapyl alcohol [22]. The absorption band at 1730cm-1 and
this can be attributed to the ester carbonyl group stretching vibration. The band at 1598 cm-1 and
1588cm-1 corresponds to the −CH stretching for aromatic nucleus indicating the presence of 4-
hydroxy
benzoic
acid
matched
from
its
IR
spectrum
https://webbook.nist.gov/cgi/cbook.cgi?ID=C99967&Type=IR-SPEC&Index=0#Refs accessed

May 06, 2019). A characteristic absorption at 1120-1130cm-1 and 1070-1080cm-1 are attributed to
C-O stretching. Bands at 1270-1280cm-1 indicated ʋ(C-H) vibration for Guaiacyl ring [42] whereas
the absorption band at 1130cm-1 indicates ʋ(C-H) vibration for Syringyl ring indicating the
presence
of
syringaldehyde
matched
from
its
available
IR
spectrum
(http://spectrabase.com/spectrum/1Vjy4kueEqX?a=SPECTRUM_1Vjy4kueEqX accessed May
06, 2019). This data could determine the functional group present in the molecule and could predict
that aromatic ester derivative might be present in the isolated molecule. Absorption spectra also
indicated the presence of phthalates matched from literature reported depolymerized products of
lignin [53].
105
85
65
45
25
5
0
1,000
2,000
Wavenumber (cm-1)
3,000
4,000
L1 (Before Catalytic activity)
L1 (After Catalytic activity)
Figure 11: FTIR of Extracted Lignin (L1) before and after Catalytic Depolymerization
of Lignin

HPLC Analysis: Catalytic activity of synthesized photocatalysts was monitored by HPLC after
the depolymerization of lignin into its respective products. Synthesized photocatalysts (SK1-SK5)
were incorporated into lignin dilution in 1,4-dioxane solvent one by one under solar irradiations.
Samples collected at different time intervals characterized by UV/visible spectroscopy to find out
the value of λmax (285nm) and then analyzed by HPLC to find the type of product of lignin formed
after different time intervals of reaction. Polar acetonitrile used as a mobile phase. One of the
products was also analyzed by taking its IR spectrum after the catalytic testing for confirmation of
the depolymerization of lignin by the catalyst. SK2 and SK3 were found to be most catalytically
active for depolymerization as compared to other catalysts (SK1, SK3, and SK4) as shown in Fig.
3 hence catalytic activity by these two is further elaborated. Different parameters of
depolymerization products of lignin by photocatalyst SK2 (L1-SK2) and SK3 (L1-SK3) are
enlisted in Table 10. Concentrations depolymerization products [22] calculated after finding RF
from Eq. 3 as follows:
푃푒푎푘 퐴푟푒푎
------- (4)
퐶표푛푐푒푛푡푟푎푡푖표푛 표푓 푑푒푝표푙푦푚푒푟푖푧푎푡푖표푛 푝푟표푑푢푐푡푠 =
푅푒푠푝표푛푠푒 퐹푎푐푡표푟 (푅퐹)
Table 10: HPLC Data of Lignin Depolymerization Products after 190min of Photocatalytic
Activity by SK2 and SK3
Peak No. HPLC Data of Lignin Depolymerization Products after 1.5h of
Photocatalytic Activity by SK2 and SK3
Retention Time
Response
Concentration
L1-SK2
Intensity Peak Area
(min)
2.4
Factor (RF) (µg/mL)
1
1123
798
2695.2
1835.4
14881.46
898.38
105.6863
81.37472
170.0686
31.79887
25.5
2
2.3
22.55
87.5
3
1.87
0.63
7958
1426
4
28.24
L1-SK3

1
2
3
4
2.9
2.3
1.4
0.8
356
1032
15203
490
55.60345
202.7067
26.48649
34.38625
18.56
75
6610
350
18.5
25.01
1075
860
Lignin after depolymerization is expected to change the composition of resulting samples in very
short time interval and results in the reduction of elution time. Chromatogram of lignin taken
before activity has shown peak 4 at 3min (retention time) which shows 100µg/mL lignin shown in
Fig. S7 (supplemental data). Peaks appeared in the chromatogram of L1-SK2 after 90min of
photocatalytic activity showed more interaction with the mobile phase hence eluted early. The
concentration of major components indicated by peak 3 found to be decreased as 87µg/mL as
shown in Fig. S8 (supplemental data). Whereas, peaks appeared in the chromatogram of L1-SK3
after 90min of photocatalytic activity have shown decreased in the concentration of major
component i.e. 75µg/mL indicated by peak 2 in Fig. S9 (supplemental data). Less elution time of
the samples L1-SK2 and L1-SK3 and decrease in concentrations after catalytic activity testing
shows the lignin depolymerization into simpler monomeric units further characterized by IR
spectrum.
Photocatalytic activity of SK2 to depolymerize the lignin sample at different time intervals also
shows the decrease in elution time which indicates the depolymerization of the original sample as
depicted in Fig. S10 (supplemental data). Elution was more rapid for the samples taken after 90min
of catalytic activity indicating the extent of depolymerization at different time intervals. This
sample after activity was confirmed by taking the IR spectrum indicating different phthalates,
Syringyl, and guaiacyl ring moieties present [53]. HPLC of L1-SK3 at different time intervals