Selective conversion of xylose to lactic acid over metal-based Lewis acid supported on γ-Al2O3 catalysts

Article abstract of DOI:10.1016/j.cattod.2020.04.061

Gamma-alumina (γ-Al₂O₃) is a low-cost amphoteric solid catalyst which can enhance the transformation of D-xylose into lactic acid. The deposition of metal oxides on γ-Al₂O₃ could further improve the yield of lactic acid from xylose. Therefore, in this work, the thermochemical catalytic conversion of D-xylose to lactic acid using the Cr, Cu, Co, Ni, and Sn oxides supported on γ-Al₂O₃ as heterogeneous catalysts was studied. The effects of metal oxides on the D-xylose conversion to lactic acid activity of the catalysts were investigated. It was found that, under identical testing conditions (170 °C, 4 h), Cr/Al₂O₃ is superior to the other catalysts with the 99 % conversion of D-xylose and the lactic acid yield of 74%. The outstanding activity could be attributed to its richness in Lewis-acid sites. Since the highly active Cr/Al₂O₃ catalyst is composed of the earth-abundant materials and can be prepared by a simple process, it has feasibility for industrial-scale application.

Full text of DOI:10.1016/j.cattod.2020.04.061

Journal Pre-proof
Selective conversion of xylose to lactic acid over metal-based Lewis acid
supported on ␥-Al O catalysts
2
3
Chanokporn Kosri, Sirapassorn Kiatphuengporn, Teera Butburee,
Saran Youngjun, Sutarat Tongrutkaew, Kajornsak Fuangnawakij,
Chakrit Yimsukanan, Narong Chanlek, Pinit Kijkhuntod, Jatuporn
Wittayakun, Pongtanawat Khemthong
PII:
S0920-5861(20)30270-4
DOI:
https://doi.org/10.1016/j.cattod.2020.04.061
CATTOD 12840
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
31 December 2019
1 April 2020
30 April 2020
Please cite this article as: Kosri C, Kiatphuengporn S, Butburee T, Youngjun S, Tongrutkaew
S, Fuangnawakij K, Yimsukanan C, Chanlek N, Kijkhuntod P, Wittayakun J, Khemthong P,
Selective conversion of xylose to lactic acid over metal-based Lewis acid supported on
␥
-Al O catalysts, Catalysis Today (2020), doi: https://doi.org/10.1016/j.cattod.2020.04.061
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1
Selective conversion of xylose to lactic acid over metal-based Lewis acid
supported on γ-Al
2
O catalysts
3
1
2
2
2
Chanokporn Kosri , Sirapassorn Kiatphuengporn , Teera Butburee , Saran Youngjun , Sutarat
2
2
2
3
Tongrutkaew , Kajornsak Fuangnawakij , Chakrit Yimsukanan , Narong Chanlek , Pinit
3
1,*
2*
Kijkhuntod , Jatuporn Wittayakun , and Pongtanawat Khemthong
1School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon
Ratchasima, Thailand
2Nanotechnology Center (NANOTEC), National Science and Technology Development Agency
(NSTDA), Pathum Thani, Thailand
3Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand
*
Corresponding authors: pongtanawat@nanotec.or.th and jatuporn@g.sut.ac.th
Graphical abstract

2
Highlights



Deposition of Cr, Co, Cu, Ni, or Sn oxides on γ-Al
The acidity of the catalysts influences the lactic yield.
Cr/Al and Sn/Al are promising catalysts for lactic acid production.
The low-cost catalysts are simply prepared by impregnation technique.
Cr/Al and Sn/Al catalysts have the potential for industrial-scale application.
2
O can promote Lewis acid sites.
3
2
O
3
2
O
3


2
O
3
2
O
3
Abstract
Gamma-alumina (γ-Al
2
O
3
) is a low-cost amphoteric solid catalyst which can enhance the
could
transformation of D-xylose into lactic acid. The deposition of metal oxides on γ-Al
2
O
3
further improve the yield of lactic acid from xylose. Therefore, in this work, the thermochemical
catalytic conversion of D-xylose to lactic acid using the Cr, Cu, Co, Ni, and Sn oxides supported
on γ-Al
conversion to lactic acid activity of the catalysts were investigated. It was found that, under
identical testing conditions (170 °C, 4 h), Cr/Al is superior to the other catalysts with the 99 %
conversion of D-xylose and the lactic acid yield of 74%. The outstanding activity could be
attributed to its richness in Lewis-acid sites. Since the highly active Cr/Al catalyst is composed
2
O
3
as heterogeneous catalysts was studied. The effects of metal oxides on the D-xylose
O
2 3
2
O
3
of the earth-abundant materials and can be prepared by a simple process, it has feasibility for
industrial-scale application.
Keywords: Lactic acid; Xylose; Al
2
3
O , Heterogeneous catalyst; Metal oxides.

3
1
. Introduction
Lactic acid is one of the essential biomass-derived platform chemicals which is used in
various industrial applications. For example, it is used as an acidulant and preservative agent in
the food and beverage industries. The esters of lactate salts with longer chain fatty acids are used
as emulsifying agents (e.g., calcium and sodium stearoyl-2-lactylate) in food and bakery goods
[1]. Moreover, lactic acid is a precursor in the synthesis of several value-added chemicals, and an
ingredient in the production of cosmetics, pharmaceuticals, and biodegradable polymers [1, 2].
Currently, lactic acid is mainly produced by fermentation of carbohydrates in the presence of
microorganisms [2]. However, high nutrient costs, low volumetric productivities, long production
periods with low space-time yields and complicate purification processes limit its upscaling
production [3]. Thus, non-fermentative approaches such as thermochemical conversion of biomass
with acid catalysts have increasingly received attention [4, 5].
There are a number of reports on thermochemical catalytic conversion of triose (glycerol,
dihydroxyacetone and glyceraldehyde) [6] and hexose (fructose and glucose) sugars to lactic acid
by Lewis acid catalysts [3, 7, 8]. However, there are only a few reports on the conversion of
pentose sugars, especially xylose [9-11] which is one of the most abundant components derived
from lignocellulose. For example, Takagaki et al. [12] have demonstrated that solid Lewis acid
catalysts with a high content of aluminum, especially gamma-alumina (-Al
selectivity of lactic acid in the transformation of C3 and C6 sugars in water. Gamma-alumina (-
Al ) is widely used as both a Lewis acid catalyst and catalyst support in many reactions [10, 11,
3-15], due to its large surface area with bi-functional acid-base properties, high stability, and low
cost.
2
3
O ), could yield high
2
O
3
1

4
Many metal-based Lewis acids such as tin chlorides (SnCl
sulfate (CuSO ) [18], cobalt sulfate (CoSO ) [18], nickel sulfate (NiSO
chlorides (CrCl and CrCl ) and chromium sulfate (Cr (SO ) [20] have been reported as the
2
and SnCl
4
) [16, 17], copper
4
4
4
) [18, 19], chromium
2
3
2
)
4 3
active catalysts for converting sugars to lactic acid. Different types of metal cations possess
different catalyst properties, such as acidity, acid site, metal-support interaction, or oxygen
adsorption capacity [21]. Moreover, the deposition of metal oxides supported on γ-Al
further enhance the D-xylose conversion and yield of lactic acid. Therefore, the composites of -
Al and metal-based Lewis acids could be potentially promising heterogeneous catalysts for the
transformation of D-xylose to lactic acid. In this work, we study the effects of various metals
cations dispersed on -Al support (M/Al where M = Co, Cu, Ni, Cr, Sn) on thermocatalytic
D-xylose-to-lactic acid conversion in aqueous solution.
2
O
3
could
2
O
3
2
O
3
2
O
3
2
2
. Experimental
.1 Preparation of the metal-based Lewis acid supported on γ-Al
2
O
3
γ-Al
at 450 °C for 3 h under air atmosphere with a ramp rate of 5 °C/min [22]. In the typical incipient
wetness impregnation, the solution of 10 wt. % of the metal precursors, including Co(NO ·6H
99%, Ajax Finechem), Cu(NO ·3H O (99%, Ajax Finechem), Ni(NO ·6H O (99%, Ajax
Finechem), Cr(NO ·9H O (99%, HIMEDIA), and SnCl ·2H O (99%, Sigma Aldrich) were
added dropwise on γ-Al under stirring. After that, the mixture was vacuum dried overnight at
0 °C. The obtained powder was then thoroughly ground and calcined at 550 °C in a muffle furnace
2
3
O was prepared by calcination of boehmite, (γ-AlO(OH)), SASOL Germany GmbH)
3
)
2
2
O
(
3
)
2
2
3
)
2
2
3
)
3
2
2
2
2
O
3
6

5
for 4 h with a ramp rate of 5 °C/min. The final catalysts were assigned as M/Al
the metal precursors (M = Co, Cu, Ni, Cr, Sn).
2
O
3
depending on
2
.2 Characterization of the catalysts
The crystalline phases of γ-Al
ray diffraction (XRD) on a Bruker D8 ADVANCE with a monochromatic light source Cu K
2
O
3
support and M/Al
2
O catalysts were characterized by X-
3
α
radiation (λ = 1.5418 Å) operated at the voltage and current of 40 kV and 40 mA, respectively.
The samples were scanned at the 2θ range of 10º < 2휃 < 80° with the scan speed of 0.5 s/step and
the increment of 2.0 s/step.
The porosity of the samples was analyzed by nitrogen sorption on a NOVA 2000e Surface
Area and Pore Size Analyzer. Before the measurement, the samples were degassed under a vacuum
at 250 °C for 2 h. The specific surface area was calculated by the Brunauer-Emmett-Teller equation
(
BET) while the pore size distribution was determined by the Barrett-Joyner-Halenda method
BJH). The acidity of the samples was determined by ammonia temperature-programmed
(
desorption (NH -TPD) on a Belcat-B equipped with a thermal conductivity detector (TCD).
3
Morphology of the samples was observed by scanning electron microscopy (SEM) on
JOEL/JSM-6010LV operated at 20 kV. The samples were coated with gold by sputtering
(Neocoater/mp-19020NCTR). Oxidation states, geometry and binding energies of the metals and
metal-γ-Al interaction were investigated by X-ray absorption spectroscopy (XAS) and X-ray
2
O
3
photoelectron spectroscopy (XPS) at Beamline 5.2 & 5.3 of the Synchrotron Light Research
Institute, Thailand.
2
.3 Evaluation of catalytic performance

6
A 125 ml stainless-steel batch reactor equipped with a Teflon liner (75 mL) was used as a
reactor for evaluating the catalytic performance of the catalysts. Firstly, the reactor was filled with
3
0 mL of DI water. Then, 0.9 g of D-xylose (98%, Sigma Aldrich) was added under stirring to
obtain the final concentration of 0.2 M. The desired amount of the catalysts (0.1, 0.5 and 1.0 g)
was then added into the D-xylose solution with continuous magnetic stirring. After that, the reactor
was sealed and purged with nitrogen gas to remove air, and then pressurized to 15 bar. The reaction
was performed at the temperature range of 130-190 °C for 6 h. When the reaction was completed,
the autoclave reactor was quenched in an ice bath and the solid catalyst was separated by filtration
through a 0.22 µm Nylon filter membrane. The liquid product was analyzed by high performance
liquid chromatography (HPLC, Shimadzu) equipped with a UV-Vis detector and a refractive index
(RI) detector. Aminex HPX-87H column from Bio-Rad was employed for product analysis using
5
mM H SO as a mobile phase with a flow rate of 0.6 mL/min at a column temperature of 45 °C.
2
4
The conversion and product yield were calculated by equations 1 and 2:
Initial mole of reactant - Final mole of reactant
Conversion (%) = ꢀ
ꢁ × 100 …………………………..(1)
Initial mole of reactant
[
P] × 푉푓 ÷ 푀푊 × Cpn
Yield (%) = ꢀ
xyloseꢁ × 100………………………………………(2)
Mass of xylose × Crn atoms ÷ 푀푊
where, [P] = concentration of product from HPLC, V
f
= final volume of the reaction, M
W
=
molecular weight of each compound, Cpn = number of carbon atoms of each product and Crn =
number of carbon atoms of reactant.
3
3
. Results and discussion
.1 Characterization of the catalysts

7
3
.1.1 Crystal structure and textural property
The crystalline phases of γ-Al
2
O
3
and M/Al
2
O
3
catalysts were characterized by powder
XRD patterns as shown in Fig. 1. The diffraction pattern of Al
characteristic peaks of the gamma-phase (PDF 00-001-1303). This result confirms a
transformation of boehmite to γ-Al by calcination at 450 °C. For the metal-loaded catalysts,
the characteristic peaks of γ-Al are still observed indicating that the gamma-phase of Al
2
3
O is consistent with the
2
O
3
2
O
3
2
O
3
remained unchanged after calcination. In addition, the metal-loaded catalysts exhibit the
characteristic diffraction patterns of the individual metal oxide species. Namely, the characteristic
peaks of Co
PDF 01-077-7716) are observed on Co/Al
However, there is an exception that the peaks of chromium oxides are not observed from Cr/Al
According to the literature, Cr(NO ·9H O can be transformed to chromium oxide (Cr ) on γ-
Al by calcination at 550 °C [22, 23]. In this case, the chromium oxide species may be in an
amorphous phase or well dispersed on the support.
3
O
4
(PDF 00-043-1003), NiO (PDF 01-081-0711), SnO
2
(PDF 01-070-6153) and CuO
(
2
O
3
, Ni/Al , Sn/Al
2
O
3
2
O
3
, and Cu/Al , respectively.
2
O
3
2
3
O .
3
)
2
2
2
O
3
2
O
3
Fig. 1.
The nitrogen adsorption-desorption isotherms and pore size distribution of γ-Al
2
3
O and
M/Al
2
O
3
catalysts are shown in Fig. 2. The specific surface area and total pore volume are listed
in Table 1. In Fig. 2A, the isotherms of all samples exhibit type IV isotherm with H1 hysteresis
loop type according to the recent IUPAC classification [24]. The hysteresis loop from the
condensation step at a relative pressure range of P/P = 0.6 – 0.9 indicates the existence of non-
0

8
uniform mesopores in the samples with a regular tubular porosity. Besides, the pore sizes of all
samples are similar, with the average of 7.7 nm. As indicated in Table 1, the BET surface area of
2
2
as-prepared γ-Al
The difference might be attributed to the difference in the starting precursors. Furthermore, the
BET surface areas of all M/Al catalysts are similar and slightly smaller than that of the γ-Al
support. These results suggest that the addition of metal oxides has a minor effect on the textural
properties of the γ-Al support The comparable surface area and pore size of the metal-loaded
catalysts could ensure that the catalytic performance is resulted from the metal species, not the
2
O (189 m /g) is lower than that in some other literature (230-250 m /g) [22, 25].
3
2
O
3
2
O
3
2
O
3
.
textural properties of γ-Al
2
3
O support.
Fig. 2.
Table 1
SEM images of all samples (Fig. 3) demonstrate the unchanged morphology of γ-Al
For all M/Al samples, small particles decorated on the surface of γ-Al with good dispersion
are observed. It can be concluded that the morphology of γ-Al did not change after the catalyst
2
3
O .
2
O
3
2
O
3
2
O
3
preparation. Notably, small nanoparticles are uniformly distributed in the mesoporous channels
and isolated from each other.
Fig. 3.
3
.1.2 Acidic property

9
Surface acidic properties of γ-Al
ammonia temperature program desorption (NH
of γ-Al consists of three peaks around 178, 350 and 483 °C corresponding to the weak, medium
and strong acid sites, respectively. Compared to the NH -TPD profile of γ-Al , the intensity of
2
O
3
and various M/Al
2
O
3
catalysts were investigated by
3
-TPD) apparatus. In Fig. 4, the NH -TPD profile
3
2
O
3
3
2
O
3
the peaks representing the weak (178 °C), and medium (350 °C) acid sites are significantly
increased, while the intensity of the peak representing strong acid sites (483 °C) is decreased in all
cases of M/Al
of M/Al catalysts can be removed easier at the lower temperatures compared to that on γ-Al
In other words, introducing metal oxides into γ-Al could increase the amount of weak acid sites
and, in contrast, decrease the amount of strong acid sites. Especially, Cr/Al exhibited the
highest amount of weak and medium acid sites, and the peak assigned to the medium acid sites is
shifted to the lower temperature (~280 °C). The order of the total acidity of supported M/Al
catalysts are Cr/Al > Sn/Al > Ni/Al > Cu/Al > Co/Al . The involvement of the
2
O
3
catalysts. These results indicate that the adsorbed NH
3
molecules on the surface
2
O
3
2
O
3.
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
2
O
3
acid sites in the catalytic reaction will be discussed in the next section.
Fig. 4.
Table 2
3
.1.3 Oxidation and chemical states

1
0
X-ray photoelectron spectroscopy (XPS) was employed to identify the chemical states and
the elemental composition of the supported M/Al catalysts (Fig. 5). Fig. 5A is the XPS spectra
of Co/Al , showing two major peaks at 781.03 and 796.20 eV, which are corresponding to the
Co 2p3/2 and Co 2p1/2 spin-orbit peaks of Co , respectively [26] along with the satellite peaks at
87.59 eV and 803.24 eV. The XPS spectrum of Cr/Al (Fig. 5A ) displays two peaks at 576.82
eV and 586.71 eV which are corresponding to Cr 2p3/2 and Cr 2p1/2 of Cr , respectively. The peaks
2
O
3
1
2
O
3
3
O
4
7
2
O
3
2
3+
6
+
at 579.54 eV and 588.82 eV correspond to Cr 2p3/2 and Cr 2p1/2 of Cr [22]. These predominant
3
+
6+
peaks represented the existence of both Cr and Cr species on the surface of Cr/Al
2
3
O . Thus,
3+
6+
chromium oxides could be in the form of Cr
2
O
3
and CrO . The ratio of Cr to Cr was 1.12
3
indicating that the amount Cr and CrO on the surface are nearly the same.
2
O
3
3
In the case of supported Ni/Al
2
O
3
catalyst (Fig. 5A ), the two main peaks at 856.87 eV
3
and 874.56 eV correspond to Ni 2p3/2 and Ni 2p1/2. Moreover, there are shake-up satellite peaks at
8
62.97 eV and 880.93 eV. There are several works reported that the binding energies of Ni 2p3/2
0
at approximately 851.7-853 eV are attributed to Ni while those at about 852.6 – 853.7 eV, 853.6
–
855.5 eV, 855.8 – 856 eV and 855.9 – 857 eV are ascribed to Al
respectively [27]. The XPS and XRD results from this work confirm that the nickel oxide species
of supported Ni/Al is NiO.
3
Ni, NiO, Ni
2
O
3
, and NiAl
2
O
4
,
2
O
3
XPS spectrum of Sn/Al
87.38 eV and 495.85 eV, corresponding to Sn species. According to the literature, the peaks of
Sn/Al are from Sn with tetragonal coordination to the oxygen atoms of the host material [28,
9]. XPS spectrum of Cu/Al (Fig. 5A ) contains multi peaks corresponding to Cu 2p3/2 and Cu
1/2 accompanied by the satellite peaks. The peaks at 933.34 eV and 953.08 eV are the
O
2 3
(Fig. 5A
4
) shows the two peaks with the central position at
4
+
4
2
O
3
2
2
2
O
3
5
p
characteristics of Cu 2p3/2 and Cu 2p1/2 peaks of CuO [30]. Moreover, the small peaks at 935.32

1
1
eV and 955.32 of Cu 2p3/2 and Cu 2p1/2 are observed, suggesting the existence of Cu(OH)
trace amount [31] However, the appearance of Cu(OH) could be ascribed to water adsorption on
the surface of Cu/Al
2
in a
.
2
2
3
O .
The O1s and Al 2p regions of γ-Al
analyzed as shown in Fig. S1 in the Supplementary Materials. The XPS spectrum of the Al
displays the characteristic peak of O 1s and Al 2p at an approximate peak position of 531 eV and
4 eV, respectively. Comparing with the supported M/Al catalysts, the XPS spectrum of O 1s
has a peak center in the range of 531-532 eV, while the peak centered at 74-75 eV is ascribed to
Al 2p associated with γ-Al [32]. However, these spectra consist of two fitting peaks which
2
O
3
and supported M/Al
2
O
3
catalysts were further
2
O
3
7
2
O
3
2
O
3
could be ascribed to Al-O-Al and Al-OH species. Thus it can be noted that these peaks could be
referred to various chemical compositions of Al and its hydroxyl groups, such as AlO(OH),
AlO(OH)
2
, Al(OH) [33]. The presence of Al-OH species was due to adsorbed water.
3
The supported M/Al
on the geometry and oxidation states. The XANES measurements were performed at the K-edge
of copper, cobalt, chromium and nickel, and at the L -edge of Sn. The XANES results confirm that
the metal species are in oxide forms. This results are in good agreement with the XRD and XPS
results. Hence, it is reasonable to deduce that the dominant metal oxide species of Co/Al
Cr/Al , Ni/Al , Sn/Al , and Cu/Al are Co , mixed Cr and CrO , NiO, SnO and
2
O
3
samples were further investigated by XANES for the information
3
2
3
O ,
2
O
3
2
O
3
2
O
3
2
O
3
3
O
4
2
O
3
3
2
CuO, respectively.
Fig. 5.

1
2
3
.2 Catalytic performance of M/Al
2
3
O for lactic acid production
3
.2.1 Effects of metal oxide suppoted on γ-Al
2
O
3
The supported M/Al
2
O
3
catalysts and Al
2
3
O support were tested for lactic acid production
from xylose at 170 °C for 6 h. The catalytic activity is shown in Fig. 6. For all catalysts, xylose
conversion increases with respect to the reaction time and nearly completed (99% conversion)
within 4 hours. The main product is lactic acid, and the by-products are furfural and formic acid.
The order of lactic acid yields derived from M/Al
Ni/Al ~ Al > Co/Al 3. This trend is consistent with the acidity in the weak acid range.
Yang et al. [11] have reported that catalytic performance of Zr-SBA-15 for the transformation of
xylose to methyl lactate increases with the increasing acidity. In this work, Cr/Al andSn/Al
provide better results than the others with the yields of 74% and 66%, respectively. On the other
hand, the lowest lactic acid yield is obtained from Co/Al , corresponding to the lowest acidity
2
O
3
is Cr/Al
2
O
3
> Sn/Al
2
O
3
> Cu/Al
2
3
O >
2
O
3
2
O
3
2
O
2
O
3
2
O
3
2
O
3
in the weak acid range. It should be noted that the amount of weak acidity has an influence on the
catalytic activities in our reaction condition. The reaction pathways for lactic acid production from
different carbohydrate substrates seem similar.
The steps in the transformation of xylose to lactic acid in aqueous solutions are
proposedand illustrated in Scheme 1. First, xylose undergoes retro-aldol condensation which is
the breaking of C–C bond between C-2 and C-3 carbons to obtain triose species (C
3
) and
glycolaldehyde (C ). The triose species could be glyceraldehyde from the retro-aldol condensation
2
of acyclic xylose as aldopentose, and dihydroxyacetone from the retro-aldol condensation of
xylulose as ketopentose from isomerization of xylose [8]. There is evidence of xylulose in our
results. However, the glyceraldehyde can be isomerized to dihydroxyacetone [14] and dehydrated
to 2-hydroxypropenal, then to pyruvaldehyde through keto-enol tautomerization, and finally to

1
3
lactic acid through intramolecular Cannizzaro reaction [10]. It has been discussed that the Lewis
acid plays an important role in the key step of retro-aldol condensation of xylose to provide
intermediates and the intramolecular Cannizzaro reaction of pyruvaldehyde to produce lactic acid
selectively [10, 11, 26]. The catalytic results show that the higher lactic acid yields are obtained
from the supported M/Al
2
O
3
which have a higher weak acidity than the bare Al
2
3
O . This indicates
that the addition of Cr, Sn and Cu could enhance weak acidity which relates to the increase of
lactic acid yield.
Scheme 1.
Moreover, furfural is produced via dehydration of xylose while formic acid is formed by
the decomposition of glycoladehyde [10] or from the C–C splitting of pyruvaldehyde or
glyceraldehyde [14] during the transformation of xylose to lactic acid.
Fig. 6.
3
.2.3 Effect of reaction conditions over Cr/Al
2
O
3
and Sn/Al
2
O
3
The outstanding catalytic performance of Cr/Al
2
O
3
and Sn/Al
2
3
O were further investigated
under the temperature range of 130 to 190 °C at a reaction time of 4 hours (Fig. 7). Those catalysts
show D-xylose conversion approximately 80% at 130 °C and then increase with increasing
temperature up to 170 °C. At this temperature, the conversion could reach 99% and become
constant at 190 °C. For lactic acid, both catalysts provide negligible yield at 130 °C. Higher yields
are obtained from the higher temperature and the highest yield is obtained at 170 °C. However, the

1
4
lactic acid yield from both catalysts decrease at 190 °C because lactic acid further transforms to
other small organic acids [34]. Otherwise, the small organic acids could be produced from C–C
splitting of pyruvaldehyde or glyceraldehyde which are active intermediate in this reaction [14].
Fig. 7.
Furthermore, the effect of catalyst loading was also investigated on Cr/Al
2
O
3
and Sn/Al
2
3
O .
From the results, both catalysts exhibit similar trends (Fig. 8). Catalyst loading of 0.1 g is sufficient
for the complete conversion of D-xylose but the low yield of lactic acid. The high lactic acid yield
is obtained with a larger amount due to more selective sites for the reaction. Similar lactic acid
yield were obtained from 0.5 g and 1.0 g of catalyst. The product masses of lactic acid, furfural
and formic acid with the best condition are reported in Table S4 of Supplementary Information.
The best condition of 0.5g Cr/Al
2
3
O catalyst in 170 °C could provide 654 mg of lactic acid, 121
mg of furfural and 35 mg of formic acid. Thus, it can be concluded that 0.5 g catalyst is optimal
for this batch reaction.
Fig. 8.
4
. Conclusions
This work demonstrates an efficient alternative route for lactic acid production from D-
xylose. High yields of lactic acid (∼70%) at total conversion are obtained starting from D-xylose

1
5
at 170 ºC under inert conditions using the oxides of Cr and Sn supported on γ-Al
2
3
O catalysts. The
results have revealed that acid sites can enhance the selectivity of lactic production. We hope that
these abundant and low-cost catalysts, as well as the facile synthesis method, could have a strong
potential application in an industrial scale in the future.
Credit Author Statement
1
2
3
4
5
6
7
8
9
1
1
. Chanokporn Kosri : Data interpretation, Writing- Original draft preparation
. Sirapassorn Kiatphuengporn : Writing- Reviewing and Editing
. Teera Butburee : Writing- Reviewing and Editing
. Saran Youngjun: XPS measuring and analysis
. Sutarat Tongrutkaew : Experimental and interpretation
. Kajornsak Fuangnawakij : Reviewing
. Chakrit Yimsukanan : Data analysis and graphic design
. Narong Chanlek : XPS analysis
. Pinit Kijkhuntod: XAS analysis
0. Jatuporn Wittayakun : Writing- Reviewing and Editing
1. Pongtanawat Khemthong : Conceptualization, Data analysis, Writing- Reviewing and
Editing
Declaration of interests
☒
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.
☒
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:

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Acknowledgements
The scholarship for C. Kosri is from the Thai government under the Development and
Promotion of Science and Technology Talents (DPST) project. The authors also acknowledge
support from National Nanotechnology Center (P1752682), SPAIII–Integrated Platform: Bio-
based Materials (P1850012), and SUT-NANOTEC-SLRI Beamline 5.2 and 5.3 (Synchrotron
Light Research Institute, Thailand) for the XANES and XPS facilities.
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Fig.1

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Fig.2

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Fig.3

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Fig.4

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Fig.5

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Fig.6

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Fig.7

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Fig.8

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Scheme 1