Roles of acidic sites in alumina catalysts for efficient d-xylose conversion to lactic acid

Article abstract of DOI:10.1039/d0gc02573a

This work investigated the conversion of d-xylose to lactic acid over heterogeneous Al2O3 and ZSM-5 catalysts in aqueous solution. γ-Al2O3, an abundant low cost catalyst, exhibited superior lactic acid yield (63 mol%) at 170 °C, compared to α-Al2O3, ZSM-5 catalysts and the reported data in the literature so far. Our experiment suggested that the outstanding lactic acid yield could be attributed to the large specific surface area and the abundance of Lewis acid sites in γ-Al2O3. In contrast, furfural selectivity was significantly promoted in the catalyst free system and the ZSM-5 catalyst, which has abundant Br?nsted acid sites, but lacks Lewis acid sites. The theoretical part revealed that reactive Lewis acid sites in γ-Al2O3 and the solvent play important roles in the C-C bond activation and thermodynamic stability of the d-xylose to lactic acid pathway. The green catalytic system proposed in this work shows great potential as an alternative method for lactic acid production for future industrial application. This journal is

Full text of DOI:10.1039/d0gc02573a

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Roles of acidic sites in alumina catalysts for
efficient D-xylose conversion to lactic acid†
Cite this: DOI: 10.1039/d0gc02573a
Sirapassorn Kiatphuengporn,a Anchalee Junkaew, *a Chuleeporn Luadthong,a
Sutarat Thongratkaew,a Chakrit Yimsukanan,a Siripit Songtawee,a Teera Butburee,a
Pongtanawat Khemthong, a Supawadee Namuangruk, a Manaschai Kunasetha,b
a
and Kajornsak Faungnawakij
*
This work investigated the conversion of D-xylose to lactic acid over heterogeneous Al2O3 and ZSM-5 cat-
alysts in aqueous solution. γ-Al2O3, an abundant low cost catalyst, exhibited superior lactic acid yield
(63 mol%) at 170 °C, compared to α-Al2O3, ZSM-5 catalysts and the reported data in the literature so far.
Our experiment suggested that the outstanding lactic acid yield could be attributed to the large specific
surface area and the abundance of Lewis acid sites in γ-Al2O3. In contrast, furfural selectivity was signifi-
cantly promoted in the catalyst free system and the ZSM-5 catalyst, which has abundant Brønsted acid
sites, but lacks Lewis acid sites. The theoretical part revealed that reactive Lewis acid sites in γ-Al2O3 and
the solvent play important roles in the C–C bond activation and thermodynamic stability of the D-xylose
to lactic acid pathway. The green catalytic system proposed in this work shows great potential as an
alternative method for lactic acid production for future industrial application.
Received 27th July 2020,
Accepted 22nd September 2020
DOI: 10.1039/d0gc02573a
rsc.li/greenchem
mainly produced by the biological fermentation of carbo-
hydrates such as xylose, glucose, sucrose, or lactose.20–22
1. Introduction
Lignocellulosic biomass is one of the most abundant feed- However, the biotechnological processes have several disad-
stocks for building a sustainable society based on a circular vantages, such as slow reaction kinetics, difficult biological
economy model. It is mainly composed of cellulose (40–50%), separation and the requirement of a large number of neutraliz-
hemicellulose (25–35%) and lignin (15–20%), while its compo- ing agents for pH control in fermentation media, making
sition can be varied depending on the sources of biomass.1–3 them difficult to scale up.10,23,24 For these reasons, chemical
Hemicellulose, the second most abundant polymer in nature, catalytic syntheses have been proposed as alternative methods
has D-xylose as the major monosaccharide building block.4,5 for lactic acid production.20,25 Both homogeneous and hetero-
For this reason, D-xylose is considered as one of the most geneous chemical catalytic systems11,12,26 have been utilized
promising cheap feedstocks for the production of fine chemi- for lactic acid synthesis.
cals and fuels.6–9 New efficient catalysts and cost-effective
There are still many challenges and comprehensive investi-
viable processes for converting D-xylose to various valuable gation is required for the development of catalysts for lactic
chemicals such as lactic acid,3,10–12 furfural,12–15 and levulinic acid production. Lactic acid can be produced from different
acid9 have been intensively explored.
substrates such as hexose or pentose sugars.27–29 The Lewis
Lactic acid is a valuable platform chemical for several com- and Brønsted acid sites were proposed to jointly act in convert-
modities, i.e. food, pharmaceutical products, etc., and various ing sugars to lactic acid over carbon–silica catalysts.29 Solvents
chemical products such as biodegradable fibers and acrylic also played a decisive role in lactic acid and lactate formation
acid.3,12,16–19 Currently, the commercial lactic acid can be over Sn-Beta catalysts.28 Yang et al. reported that calcium
hydroxide (Ca(OH)2) is an active homogeneous catalyst for con-
verting xylose to lactic acid, achieving 57% selectivity among
aNational Nanotechnology Center (NANOTEC), National Science and Technology
Development Agency (NSTDA), Pathum Thani 12120, Thailand.
E-mail: kajornsak@nanotec.or.th, anchalee@nanotec.or.th
bThaiSC, National Electronics and Computer Technology Center (NECTEC), National
Science and Technology Development Agency (NSTDA), Pathum Thani 12120,
Thailand
saccharinic acid products at a reaction temperature of 200 °C
for 10 minutes.26 Yang et al. revealed that the homogeneous
NaOH catalyst provided 74% yield of lactic acid.11 However,
these homogeneous catalysts are corrosive to the reactor and
they are difficult to be recovered from the reaction media.30
Heterogeneous catalysts have been proposed to overcome
these drawbacks.16 Nevertheless, only a few reports on hetero-
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0gc02573a
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geneous catalysts for D-xylose conversion to lactic acid have
been published so far. For example, Onda et al. reported that
the heterogeneous Pt/C catalyst in NaOH aqueous solution
2. Experimental and theoretical
procedures
(pH = 14) produced only 27% yield of lactic acid at 80 °C for 2.1. Catalyst preparation
2 h.10 ZrO2 and MgO catalysts converted xylose to lactic acid
2.1.1. Alumina. The commercial gamma-alumina (γ-Al2O3)
with 40% yield at the reaction temperature of
200–220 °C.11,31 Recently, LaCoO3 perovskite metal oxide has
been reported as a redox catalyst for the D-xylose conversion
to lactic acid. Approximately 38% yield of lactic acid was
obtained when the reaction was performed at 200 °C for one
hour under a 28 bar N2 atmosphere.32 To the best of our
knowledge, heterogeneous catalysts that can achieve more
than 60% yield of lactic acid from xylose conversion have
been rarely reported.
In the literature, the conversion of D-xylose can produce
different products such as D-xylulose, glycolaldehyde, glyceral-
dehyde, dihydroxyacetone, pyruvaldehyde, furfural, lactic acid
and formaldehyde via different pathways. The reaction path-
ways would possibly involve many complex steps such as
retro-aldol condensation, dehydration, internal Cannizzaro
reaction and Lobry de Bruyn–Alberda van Ekenstein trans-
formation (LBET).11,33 It has been reported that Lewis acid/
base pair sites on a catalyst surface might play an important
role in some key steps which promote the D-xylose conversion
to lactic acid.11,16,29,34 We performed the preliminary tests of
comparing the catalytic conversion of D-xylose to lactic acid
over different heterogeneous catalysts such as ZSM-5, γ-Al2O3,
SiO2, TiO2, CuO and NiO. As presented in Fig. S1 in the
ESI,† γ-Al2O3 provided the highest lactic acid yield among
the tested catalysts. We also conducted the reaction on
γ-Al2O3 with different sugar substrates including xylose,
glucose and fructose, as shown in Fig. S2 in the ESI.†
Alumina exhibited promising activity in the xylose conversion
to lactic acid.
To gain more understanding, we therefore focus on the con-
version of D-xylose to lactic acid over Al2O3 in this work. The
catalytic efficiency of Al2O3, which is a well-known support/
catalyst with abundant Lewis acid sites, was tested and com-
pared with those in the ZSM-5 zeolite and catalyst-free
systems. The effects of thermal annealing on the crystal phase
transformation and the morphology of alumina nanoparticles
towards the amount of acid sites and catalytic reactivity had
been comprehensively studied. Furthermore, the effects of the
operating conditions, including the amount of catalyst
loading, reaction time, reaction temperature, and catalyst re-
usability, were thoroughly investigated. Since γ-Al2O3 showed
promising catalytic performance for converting D-xylose to
lactic acid compared to ZSM-5 and other heterogeneous cata-
lysts reported in the literature, DFT calculations were per-
formed to understand the nature of its active sites. Moreover,
the roles of these active sites and solvent in the interactions
between the molecules and the catalyst surface were investi-
gated herein. Unprecedented D-xylose-to-lactic acid conversion
performance of this abundant cheap catalyst, prepared by the
powder, denoted as Al-RT (Sasol, Germany), was calcined
under ambient conditions at 500, 700 and 1100 °C for 4 h with
a heating rate of 3 °C min−1. The calcined products were
assigned as Al-500, Al-700, and Al-1100, respectively.
2.1.2. Zeolite. The commercial MFI zeolite in the
ammonium form, CBV8014 (Zeolyst International, Si/Al molar
ratio = 40), was transformed into the acid form by calcination
at 550 °C for 5 h under ambient conditions with a heating rate
of 3 °C min−1. This zeolite catalyst is denoted as ZSM-5 in this
work.
2.2. Catalyst characterization
The crystalline phases of the catalysts were identified by
powder X-ray diffraction (XRD; D8 ADVANCE, Bruker) using Cu
Kα radiation. The instrument was operated at 40 kV and 40 mA
with an increment of 0.02° per second and a step time of 0.5 s
over a range of 20° < 2θ < 70°. The specific surface area was
measured by the Brunauer-Emmett-Teller (BET) nitrogen
adsorption–desorption technique (Nova2000e, Quantachrome
Instruments). The pore size distribution was calculated by the
Barrett–Joyner–Halenda (BJH) desorption method using the
cylindrical pore model. The morphological and microscopic
details of the particles were observed by transmission electron
microscopy (TEM; JEOL JEM-2100 Plus, Japan) performed at
200 keV.
The acidic properties of the catalysts were analyzed by infra-
red-mass spectroscopy combined with temperature-pro-
grammed desorption of adsorbed ammonia (IRMS-TPD analy-
zer; Microtrac BEL). The technique is based on operando
diffuse reflectance Fourier transform spectroscopy (DRIFTS)
with NH3 probe molecules. Briefly, each sample was pretreated
in a helium flow at 550 °C for 1 h. After cooling down to 70 °C,
the sample was saturated with ammonia for 1 h. Then, helium
gas was introduced to remove physically adsorbed ammonia
molecules on the catalyst surface. Finally, ammonia desorp-
tion was carried out during the elevated temperature ranging
from 70 to 530 °C at a heating rate of 5 °C min−1 under a
helium flow. IR spectra were recorded at each 10 K, and mass
spectra were collected at m/z = 16 using a mass spectrometer
(Pfeiffer Vacuum QMS200M2). The number of each of the
Brønsted and Lewis acid sites was calculated from the intensity
of ammonia desorption in the TPD profiles following previous
studies.35,36 The amount of carbonaceous deposit on the cata-
lyst after the reaction test was determined using a carbon–
hydrogen–nitrogen elemental analyzer (CHN; CHN628, LECO,
Japan).
2.3. Evaluation of catalytic properties
facile method presented in this work, emphasized its strong In a typical experiment, a desired amount of the catalyst was
potential for application at an industrial scale.
added into a stainless steel autoclave containing aqueous
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D-xylose (0.2 M) with continuous magnetic stirring for 4 h. Dynamics (AIMD) was applied to attain the hydroxylated
Then the reactor was heated up to the required temperature surface. Under the canonical ensemble (NVT), the dynamic be-
(130–190 °C), while the pressure was controlled at 15 bar. The havior of the four water molecules on Al2O3 (110) at 300 K was
zero minute of the reaction time was recorded when the temp- determined using the Nosé–Hoover thermostat.46 The
erature reached the set point. After completing the reaction, obtained hydroxylated surface was further optimized before
the autoclave was cooled down to ambient temperature in an calculating the gas adsorption. The optimized bare γ-Al2O3
ice-water bath. Centrifugation (9000 rpm) was applied to separ- (110) and hydroxylated γ-Al2O3 (110) are depicted in Fig. 9a
ate the solid catalyst from the liquid products. The lactic acid and b, respectively.
product and by-products including furfural and formic acid
The vibrational analysis of the ammonia adsorption part
were then analyzed by high performance liquid chromato- was predicted by the Density-Functional Perturbation Theory
graphy (HPLC, Shimadzu) with a UV detector adjusted to (DFPT) method.47 For the D-xylose, D-xylulose and D-lactic acid
210 nm and a refractive index (RI) detector using an Aminex adsorbed models, the hydroxylated Al2O3 (110) slab was
HPX-87H ion-exchange column (300 mm in length with a extended to a 2 × 2 supercell to prevent the interaction
7.8 mm i.d.; Bio-Rad, Hercules, CA, USA). The column temp- between these adsorbates and their replicas. Γ-point sampling
erature was set at 45 °C. The samples were eluted with 5 mM of the Brillouin zone was used for a 15.7 Å × 16.4 Å × 22.3 Å
sulfuric acid solution in DI–water at a flow rate of 0.6 mL box. An implicit solvation model was applied in order to attain
min−1. The gaseous products were qualitatively measured by information about the solvent effect on the interaction
gas chromatography (GC, Shimadzu).
between a molecule and the surface. VASPsol,48,49 widely used
The D-xylose conversion and molar yield of aqueous-phase software in various studies,50–52 was utilized in this work.
products were calculated using the following equations based More computational details are given in the ESI.†
on the definition in the literature:11,33
ꢀ
ꢁ
Mole of D‐xylose consumed
Mole of D‐xylose initially charged
Conversion ð%Þ ¼
ꢀ 100
3. Results and discussion
3.1. Catalyst characterization
ꢀ
ꢁ
Mole of aqueous phase product
Mole of D‐xylose feedstock
Yield ðmol%Þ ¼
ꢀ 100
The crystalline phases of catalysts were characterized using the
XRD technique. As shown in Fig. 1, commercial alumina and
alumina calcined at 500 and 700 °C exhibited only a single
γ-Al2O3 phase (PDF 00-010-0425). When the calcination temp-
erature was increased up to 1100 °C, γ-Al2O3 completely faded
out, and the crystalline phase became a pure alpha-alumina
(α-Al2O3) (PDF 00-010-0173). In the case of ZSM-5, the observed
2θ values at 8.0°, 9.0°, 14.8°, 22.9°, 24.0° and 29.8° identified
Quantitative analysis of each component was accomplished
by using calibration curves. Furthermore, the reusability of the
catalyst in this reaction was tested over three consecutive
cycles without any post-treatment. After the 3rd cycle, the spent
catalyst was regenerated by calcination in air at 500 °C for 4 h
to remove carbonaceous deposits from the catalyst surface.
Then the regenerated catalyst was tested under the same reac-
tion conditions.
2.4. Computational method and models
To gain more understanding, a plane-wave based Density
Functional Theory (DFT) implemented in VASP (version
5.4.1)37,38 was used to investigate the nature of active sites on
the γ-Al2O3 surface. The Perdew–Burke–Ernzerhof (PBE)
exchange correlation functional with the projector-augmented
wave (PAW) method was utilized in this work.39,40 An energy
cutoff of 400 eV and Gaussian smearing with a σ value of 0.1
were applied. The energy and force criteria for optimization
convergence were set at 1 × 10−6 eV and 1 × 10−2 eV Å−1
respectively.
,
Typically, the (110) surface is the majority facet of γ-Al2O3.41
Thus, the γ-Al2O3 (110) model based on the work of Digne
et al.42 was used to represent the surface catalyst in this work.
From the work of Digne et al.,43 an OH coverage (θ) of 12.9 OH
per nm2 on γ-Al2O3 (110) was suggested to be present in the
range of temperatures tested in our experimental part. Due to
the complication of the hydroxylation of the γ-Al2O3 (110)
surface, different models have been proposed in previous
Fig. 1 XRD patterns of (a) commercial alumina (Al-RT), (b) calcined
alumina at 500 °C (Al-500), (c) 700 °C (Al-700), and (d) 1100 °C (Al-
◆
▼
1100), and (e) ZSM-5 catalysts: ( ) gamma alumina phase and ( ) alpha
studies for this coverage.43–45 In this work, ab initio Molecular alumina phase.
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Fig. 2 TEM images of (a) Al-RT, (b) Al-500, (c) Al-700, (d) Al-1100, and (e) ZSM-5 catalysts.
the major peaks of 101, 200, 301, 501, 303 and 503 surfaces of RT (<20 nm). This evidence suggested that the particles experi-
a typical pattern of the framework structure (MFI). enced coalescence and recrystallization during the calcination
In addition, transmission electron microscopy (TEM) and process. This sintering may result in a lower surface area of
selected area electron diffraction (SAED) were performed to the material, leading to better crystallinity as indicated by
inspect the nanostructural details at the significant points of SAED. Further details from the surface area and pore size ana-
phase changes of Al2O3. The TEM images of Al-RT, Al-500, Al- lyses are discussed.
700 and Al-1100 are presented in Fig. 2a–d, respectively. Fig. 2e
The surface area and pore size distribution of the commer-
shows the TEM image of ZSM-5 for comparison. As a result, cial alumina, calcined alumina and zeolite were evaluated by
the particle size tended to grow larger when the calcination the BET and BJH techniques, as summarized in Table 1. The
temperature was increased. The insets of Fig. 2a–c show the results showed that the Al-RT has a specific surface area of
corresponding SAED images of Al-RT, Al-500, and Al-700 (in 216 m2 g−1 with a total pore volume of 0.610 cm3 g−1 and a
the area of white circles). The SAED of Al-RT showed a ring pore diameter of 7.8 nm. On the other hand, the surface area
pattern revealing the characteristic of multi-crystalline and total pore volume significantly decreased with respect to
materials. Similarly, a ring pattern can be observed in the higher calcination temperature. The specific surface areas con-
SAED image of Al-500, but with clearer spots of the rings, indi- tinuously decreased from 202 m2 g−1 (for Al-500) to 8 m2 g−1
cating its better crystallinity than Al-RT. Apart from the rings, a (for Al-1100). A similar tendency was also observed for their
spot pattern representing the characteristic of single-crystalline pore volumes, which decreased from 0.580 cm3 g−1 (for Al-500)
structures was also observed in the SAED image of Al-700. In to 0.150 cm3 g−1 (for Al-1100). The pore diameters are 7.8 nm
Al-1100, the particle size can grow to >200 nm, compared to Al- in Al-RT, Al-500 and Al-700, but gradually increase to 98.9 nm
Table 1 Physical properties of Al2O3 and ZSM-5 catalysts
Phase of the
sample
SBET (m2
g−1
Smicro (m2
Total pore volume (cm3
g−1
Micropore volume (cm3
dBJH
(nm)
Catalysts
)
g−1
)
)
g−1
)
Al-RT
γ
γ
216
203
—
—
0.61
0.41
—
—
7.8
5.6
Al-RT (after the 1st cycle
test)
Al-RT (after the 2nd cycle
γ
γ
158
112
—
—
0.31
0.08
—
—
3.7
3.2
test)
Al-RT (after the 3st cycle
test)
Al-RT (after regeneration)
γ
γ
γ
γ
-
161
202
190
8
—
—
—
—
0.45
0.58
0.55
0.15
0.23
—
—
—
—
6.9
7.8
7.8
98.9
3.9
Al-500
Al-700
Al-1100
ZSM-5
427
252
0.13
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in Al-1100. These results suggested that the elevated calcina- that the total acid sites decreased in the following order: Al-RT
tion temperatures would lead to coalescence, sintering and > ZSM-5 > Al-500 > Al-700 > Al-1100. In the case of Al2O3 cata-
recrystallization, resulting in a significant decrease in the lysts, Al-RT has the highest amount and concentration of acid
surface area and total pore volume. These aspects were in good sites (0.52 mmol g−1 and 112.3 mmol m−2), while the lowest
agreement with the morphological observation by TEM. In the can be observed over the Al-1100 catalyst (0.05 mmol g−1 and
case of ZSM-5, the specific surface area was 427 m2 g−1 with a 0.4 mmol m−2). It should be noted that the calcination temp-
total pore volume of 0.23 cm3 g−1 and an average pore dia- erature influenced the acidity of Al2O3: the catalyst with lower
meter of 3.9 nm.
calcination temperature exhibited higher acidity. The majority
of acid sites in all alumina catalysts is Lewis acid sites. The
highest amount and concentration of Lewis acid sites were
found in Al-RT (0.34 mmol g−1 and 78.4 mmol m−2). Brønsted
acid sites (0.18 mmol g−1 and 38.9 mmol m−2) were measured
in Al-RT, while these sites in Al-500 and Al-700 were extremely
low. These Brønsted acid sites eventually disappeared in Al-
3.2. Acidic properties
The acidic properties of commercial Al2O3 and calcined Al2O3
at different calcination temperatures and ZSM-5 were
measured by IRMS-TPD. The measured amount and concen-
tration of the acid are summarized in Table 2. It was observed
Table 2 Acidic properties of Al2O3 and ZSM-5 catalysts analyzed by IRMS-TPD
Acid amount (mmol g−1
)
Acid concentration (mmol m−2
)
Catalysts
Total
Brønsted
Lewis
Total
Brønsted
Lewis
Al-RT
0.52
0.35
0.25
0.05
0.36
0.18
0.06
0.02
Not detected
0.33
0.34
0.29
0.23
0.05
0.03
112.3
70.7
47.5
0.4
38.9
12.1
3.8
78.4
58.6
43.7
0.4
Al-500
Al-700
Al-1100
ZSM-5
Not detected
140.9
153.7
12.8
Fig. 3 Acidic properties of Al-RT and ZSM-5 measured by IRMS-TPD. (a) and (c) show the estimated TPD profiles from the corresponding species
on Al-RT and ZSM-5, respectively, and (b) and (d) show the different IR spectra of Al-RT and ZSM-5, respectively, recorded at every 10 K during
temperature elevation.
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1100. In contrast, ZSM-5 possessed high amounts and concen-
tration of the Brønsted acid (0.33 mmol g−1 and 140.9 mmol
m−2) and relatively low amounts and concentration of the
Lewis acid (0.03 mmol g−1 and 12.8 mmol m−2).
Moreover, Fig. 3 shows the TPD and DRIFTS profiles of the
Al-RT and ZSM-5 catalysts that have comparable amounts of
acid sites, but are different in the nature and type of acid sites
(Lewis or Brønsted). In the Al-RT catalyst, the strong bending
vibration bands at 1245 cm−1 and 1610 cm−1 were attributed
to the adsorption of ammonia on the Lewis acid site (NH3L),
while the band at 1450 cm−1 presents ammonia at the
Brønsted acid site (NH4B).8,53 This evidence supported the
presence of both Lewis and Brønsted acid sites in Al2O3, as
shown in Table 2. In contrast, the ZSM-5 catalyst mainly
exhibited only a strong band of adsorbed ammonia on the
Brønsted acid site (NH4B) at 1450 cm−1. This result also corres-
ponds well to the observed amount of Brønsted acid sites but
lack of Lewis acid sites, as shown in Table 2.
Fig. 4 Time-dependence of D-xylose conversion (a) without the
alumina catalyst and (b) with the alumina catalyst (Al-RT). Reaction con-
According to the acidic property analysis, the alumina cata- ditions: D-Xylose solution, 0.2 M; catalyst weight, 0 or 0.5 g; reaction
temperature, 170 °C; initial N2 pressure, 15 bar and reaction time, 0–6 h.
lysts possessed dominant Lewis acid sites with partial
Brønsted acid ones – these acid sites are presumed to be the
active sites for the catalytic conversion of xylose. The inter-
action between NH3/NH4 and the active sites on the γ-Al2O3
surface was further investigated by DFT calculations. As a a high content of furfural (52.8% yield) at 6 h. The selectivity
result, NH3 adsorbs strongly on the Al4a* site with an adsorp- to furfural was typically higher than 70%. This result demon-
tion energy (Eads) of −0.92 eV, as shown in Fig. S6a in the ESI.† strated that D-xylose was preferably converted into furfural via
N of NH3 forms a chemical bond with the Al Lewis acid site dehydration reaction in the presence of high-temperature
with a bond length of 2.06 Å. Likewise, NH3 possibly adsorbs water without a catalyst, which is in good agreement with a
over the Brønsted acid site (OH) with an Eads of −0.70 eV (see previous report.33 As seen in Fig. 4b, the catalytic behavior
Fig. S6b in the ESI†). In this configuration, NH3 tends to form clearly differed in the presence of the alumina catalyst (Al-RT).
NH4 with H of the hydroxyl group of the surface. Another At 170 °C, the conversion of D-xylose reached >99% within 2 h,
possibility is the interaction between the formed NH4 and the which was much faster than the conversion rate in the blank
catalyst surface. As shown in Fig. S6c in the ESI,† hydrogen test, indicating the significant role of the surface reaction. Al-
bonds between NH4 and the surrounding OH groups result in RT could catalyze the D-xylose conversion to lactic acid with a
an Eads of −1.50 eV. The configurations in Fig. S6a–S6c† rep- high yield of 63% within 4 h, while a furfural yield of ca.
resent NH3/L, NH3/B and NH4/B, respectively.
24–25% was achieved simultaneously. A slight decrement in
The vibrational modes of these structures were further pre- the lactic acid yield after 4 h suggested that lactic acid could
dicted. Our calculated results agree well with the report from be partially converted to other products.
Suganuma et al.54 and our experiment. The vibrational modes
Next, the catalytic performances of the heterogeneous Al2O3
of NH3/L, NH4/B and NH3H contribute mostly to the broad- catalysts (γ and α phases) (i.e. Al-RT, Al-500, Al-700 and Al-
ened peak around 1600 to 1800 cm−1. The vibrational modes 1100) and the ZSM-5 catalyst were tested for the reaction. The
of NH4/B also dominate around 1450 cm−1. The wave numbers comparison of their catalytic performances at 170 °C for 4 h is
lower than 1350 cm−1 can be attributed to NH3, NH4 and O–H given in Table 3. The yields of aqueous phase products are
of the surface. The strong chemisorption of NH3 on the Lewis reported in the table. The major products including lactic
acid Al site and the tendency to form NH4 on the Brønsted acid, furfural and formic acid were detected in the aqueous
acid sites (OH) demonstrate that both types of acid sites were phase (ca. 70–90 mol% yield). The other by-products were
reactive on the hydroxylated γ-Al2O3 (110) surface.
gaseous compounds (CO and CO2) detected by GC and
unknown aqueous ones, i.e. humins. When comparing among
γ-alumina catalysts (Al-RT, Al-500 and Al-700), D-xylose conver-
3.3. Catalytic performance
3.3.1. Effects of acidic properties. The catalytic activity of sion was almost similar (>99%). For α-alumina (Al-1100), the
the heterogeneous catalysts with different acidic properties D-xylose conversion was 97.5%, which was lower than those of
was evaluated towards lactic acid production from D-xylose. γ-Al2O3 catalysts. Moreover, the calcination temperature also
Initially, a blank test without the catalyst was carried out at influenced the catalytic activity in terms of the lactic acid
170 °C for 0–6 h.
yield. It was decreased in the following order: Al-RT (63.0%) >
As shown by the results in Fig. 4a, the catalyst-free system Al-500 (54.5%) > Al-700 (50.2%) > Al-1100 (30.9%), which corre-
yielded a small amount of lactic acid in a range of 0–1.0%, but lates with their acidity shown in Table 2. The calcination temp-
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Table 3 Catalytic performance of alumina and zeolite catalysts in D-xylose conversion. Reaction conditions: D-Xylose solution, 0.2 M; catalyst
weight, 0.5 g; reaction temperature, 170 °C; initial N2 pressure, 15 bar and reaction time, 4 h
Molar yield of aqueous phase products
Catalysts
D-xylose conversion (%)
Lactic acid selectivity (%)
Lactic acid (mol%)
Furfural (mol%)
Formic acid (mol%)
Al-RT
100.0
99.9
100.0
97.5
63.0
54.6
50.3
31.7
3.4
63.0
54.5
50.2
30.9
3.1
24.5
23.8
25.7
51.4
42.4
2.0
2.1
2.4
4.5
5.2
Al-500
Al-700
Al-1100
ZSM-5
89.2
erature clearly affects the catalytic reactivity by means of chan-
ging crystallinity, acidity and surface area of Al2O3.
The correlation between the lactic acid yield and the
surface area and Lewis acid sites of Al2O3 catalysts is shown in
In the case of the zeolite catalyst, ZSM-5 gave a D-xylose con- Fig. 5. It was clearly seen that higher surface area and quantity
version of 89.2% with a lactic acid yield of 3.1 mol% and a fur- of Lewis acid sites of the catalysts could produce more lactic
fural yield of 42.4 mol%. Although ZSM-5 has the highest acid yield. The catalytic activity of Al2O3 was compared to the
specific surface area and the highest total acid amount, reported data in the literature (given in Table 4) on other
especially Brønsted acid sites, it showed lower xylose conver- tested metal oxide catalysts and zeolite (Fig. S1 in the ESI†).
sion and lactic acid yield compared to the Al2O3 catalysts. The The alumina catalyst proposed in the present work gave the
results emphasize the important role of Lewis acid sites in the maximum lactic yield of 63 mol% from xylose conversion,
xylose conversion to lactic acid.
while the reported data were in a range below 41 mol%.
According to the proposed mechanisms of D-xylose conver-
sion to lactic acid in aqueous solutions (see Fig. S3 in the
ESI†), the C–C bond breaking leading to the retro-aldol reac-
tion pathway is a key step for lactic acid production, while the
dehydration pathway was proposed for the formation of
furfural.33,57 Lewis acid sites play an important role in promot-
ing this retro-aldol pathway in the reported heterogeneous
catalysts.16,33,34 Likewise, our results indicated that the surface
area and type of surface acidic sites are essential descriptors
that control the catalytic performance in the conversion of
xylose to lactic acid. Note that we further performed the chiral
analysis using a chiral chromatography technique, and the
ratio between D- and L-lactic acid products was nearly one,
suggesting that the alumina could not selectively control the
stereoisomer of lactic acid. More details will be further dis-
cussed in the calculation section.
3.3.2. Effects of reaction temperature. To investigate the
effect of operating temperature, the catalytic reactions over Al-
Fig. 5 The correlation between lactic acid yield with surface area and
the Lewis acid sites of the Al2O3 catalyst.
Table 4 Catalytic activity of this work compared to other literature studies
Xylose conversion Lactic acid yield
Catalysts
Conditions
(%)
(%)
Reference
This work
Al-RT
170 °C; 4 h; 15 bar N2; xylose solution, 0.2 M; 0.5 g catalyst
100.0
99.9
100.0
97.5
63.0
54.5
50.2
30.9
3.1
Al-500
Al-700
Al-1100
ZSM-40
89.2
Pt/C + NaOH 80 °C; 2 h; xylose solution, 0.05 M; NaOH, 1.0 M;
100.0
27.0
Onda et al.10
Yang et al.11
50 mg catalyst; air flow, 20 mL min−1
200 °C; 40 min; 24 bar N2 + O2; xylose solution, 0.5 M;
1.2 g catalyst
220 °C; 1 h; 20 bar N2; 0.5 g catalyst
200 °C; 1 h; 27.6 bar N2; xylose solution, 0.05 M; 1.0 g catalyst
ZrO2
100.0
40.1
MgO
LaCoO3
La-HPMo-3
—
—
41.0
37.9
39.2
He et al.55
Yang et al.32
Marianou et al.56
170 °C; 1 h; 20 bar N2; xylose solution, 0.022 M; 0.07 g La-HPMo-3 catalyst 97.0
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that the activity of Al-RT continuously decreased during the
three consecutive cycles without any post-treatment. Lactic
acid yields were 63.0%, 46.7% and 30.2% for the 1st, 2nd and
3rd cycles, respectively. The XRD results of the spent catalyst
from the 3rd cycle revealed that the γ phase of alumina
remained stable, while its peak intensity slightly decreased
(see Fig. S4 in the ESI†). The physical properties of spent Al2O3
after each consecutive cycle are given in Table 1. The decrease
in catalytic activity during the three repeating cycles may be
caused by the lower surface area (112 m2 g−1) compared to that
of the corresponding fresh catalyst, probably due to the depo-
sition of some solid residues on the catalyst surface.
According to the elemental analysis, the carbon contents of
14.6, 22.7 and 26.0 wt% were detected on the spent catalysts
after the 1st, 2nd and 3rd cycles, respectively. Therefore, the
elimination of the carbonaceous deposits would be required to
reactivate the catalyst for the reuse purpose. After simple
regeneration by air calcination at 500 °C for 4 h, the catalytic
performance of the catalyst can be achieved as it showed com-
parable activity to the fresh Al-500 catalyst, which provided a
D-xylose conversion of 99.7% and a lactic acid yield of 56.0%.
Fig. 6 Catalytic performance of Al-RT in D-xylose conversion at various
temperatures. Reaction conditions: D-xylose solution, 0.2 M; catalyst
weight, 0.5 g; reaction temperature, 130–190 °C; initial N2 pressure, 15
bar and reaction time, 4 h.
RT were performed at various temperatures (i.e. 130, 150, 160,
170, 180 and 190 °C), under an identical N2 atmosphere (15
bar). The results in Fig. 6 revealed that the conversion signifi-
cantly increased with respect to the increment of the reaction
temperature. The reaction at 130 °C was likely to be ineffective
at producing lactic acid, although good conversion of D-xylose
could be achieved. The higher temperatures of 150, 160 and
170 °C gave lactic acid yields of 34.4%, 57.0% and 63.0%,
respectively. However, when the reaction temperature was
further increased to 180 and 190 °C, the yield significantly
decreased to 57.6% and 38.9%, respectively. This result might
be ascribed to the degradation of lactic acid at higher tempera-
tures in aqueous media.12 Thus, the reaction temperature of
170 °C was optimal for lactic acid production from xylose over
the Al2O3 catalyst.
3.4. Role of active sites and solvent in lactic acid production
by DFT calculations
According to the experimental results, Lewis acid sites play an
important role in the selectivity to lactic acid production.
Herein, DFT calculations were performed to attain more
understanding. First, the energy profiles of the lactic acid pro-
duction and the furfural production pathways are compared in
a catalyst-free system. The calculations were performed for the
cases under vacuum (i.e. gas phase) and in implicit water (i.e.
aqueous solution). Second, the adsorption of the reactant and
product was calculated. The influence of a solvent on the
adsorption was also investigated in this part.
3.4.1. Thermodynamic stability in the catalyst-free system.
The intrinsic thermodynamic stability of intermediates along
the lactic acid and furfural pathways was elucidated in the
catalyst-free system. The implicit solvent model was also eluci-
dated and compared to gas phase calculations. Details of solu-
tion calculations are given in the ESI.† Based on the proposed
mechanisms in the literature,33,57–59 three main pathways are
considered in this work. Fig. S3 in the ESI† presents a sche-
matic of the possible pathways consisting of (i) lactic acid pro-
duction (path A), (ii) furfural production via D-xylose (path B)
and (iii) furfural production via the isomerisation to D-xylulose
(path C). In path A, the C–C bond breaking of a C5 sugar into
a C2 intermediate (glycolaldehyde) and a C3 intermediate (gly-
ceraldehyde or dihydroxyacetone), called the retro-aldol
pathway, is the crucial step leading to lactic acid production.
Dehydration reaction to furfural can possibly proceed via two
possible pathways.58 The energy profiles in gas and solution of
these proposed pathways are compared in Fig. S8 in the ESI.†
It is to be noted that the linear chains of D-xylose and
D-xylulose presented in our work are more stabilized than the
closed form proposed in the references.58,59 In both gas and
3.3.3. Reusability of the catalyst. The catalyst reusability is
a key challenge to make the process more economically feas-
ible by reducing the operating cost in large-scale production.
Herein, the stability and reusability of the alumina catalyst (Al-
RT) was studied under the optimum conditions. Fig. 7 shows
Fig. 7 Stability and reusability of the Al-500 catalyst in D-xylose conver-
sion to lactic acid. Reaction conditions: D-Xylose solution, 0.2 M; catalyst
weight, 0.5 g; reaction temperature, 170 °C; initial N2 pressure, 15 bar
and reaction time, 4 h.
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Fig. 8 Energy profiles of the lactic acid production via (a) path A in the gas phase and (b) path A in the aqueous phase, and the furfural production
via (c) path B in the gas phase and (d) path B in the aqueous phase (ε of 39.9).
aqueous phases (at room temperature), path B1 is more only stabilizes the surface, but also plays a role in the Lewis
thermodynamically favorable than other pathways (see Fig. S8 acidity of Al and basicity of the surface. The hydroxylated
in the ESI†). This result confirms that dehydration via deoxy- surface (θ of 12.4 OH per nm2) attained by the AIMD method
xylose (INTb1) is more preferable than dehydration via xylulose is shown in Fig. 9b. All four water molecules are dissociated
in the catalyst-free system, as proposed by Hu et al.58
and they formed hydroxyl groups on the surface. The Al sites
Paths A and B1 were further investigated at 170 °C, the on the hydroxylated surface are labelled by an asterisk symbol
optimum temperature in our experiment. A dielectric constant (* or **). The optimized hydroxylated surface in our work
(ε) of 39.9 was used to represent the aqueous media at 170 °C agrees well with the model proposed by Wischert et al.60 On
(see details in the ESI†). In Fig. 8, the energy profiles of paths the hydroxylated γ-Al2O3 surface, the Al- and OH-sites rep-
A and B1 in gaseous and aqueous phases are compared. The resent the Lewis- and Brønsted-acid characteristics,
reference states, IS1 and IS1′, are D-xylose in gaseous and respectively.
aqueous phases, respectively. According to the negative sol-
The adsorption of reactants and products was calculated to
vation energy (Esol) shown in Table S1 in the ESI,† an implicit understand the interaction between those molecules and the
solvent can stabilize the intermediates in all pathways. In active sites of the catalyst surface. It is worth noting that the
Fig. 8a and b, the solvent shows a small effect on path three-dimensional structures of D- and L-lactic acids optimized
A. Similar to gas phase calculations, most elementary steps in in the gas phase are similar and their total energies are identi-
path A tend to be endodermic reactions in aqueous phase. cal (see Fig. S7 and Table S1 in the ESI†). Their conformational
Interestingly, the solvent can stabilize the intermediates and similarity and energetic stability result in the poor selectivity
products in path B1 and this pathway becomes an exothermic control of the stereoisomer of lactic acid on γ-Al2O3, observed
reaction. This evidence supports our experimental result that in our chiral analysis. In this work, only D-lactic acid adsorp-
the furfural production pathway is predominant in the cata- tion was investigated. Moreover, the adsorption of D-xylulose
lyst-free system, as shown in Fig. 4a.
was also tested, because D-xylulose was proposed as one of the
3.4.2. Adsorption on the Al2O3 surface. As shown in Fig. 9a, key intermediates in the lactic acid production pathway (see
a bare γ-Al2O3 (110) 1 × 1 surface consists of one tri-co- Fig. S3 in the ESI†). The effect of solvent on the adsorption is
ordinated Al site (Al3) and three tetra-coordinated Al sites (two also discussed in this part.
Al4a sites and one Al4b site). In the literature, water dissociation
The stable structure of each adsorbed model is shown in
was found to be facile on those highly active Al and O sites; Fig. 9. The isolated molecules under vacuum and their
therefore, the hydroxylation can be observed on the γ-Al2O3 adsorbed configurations using the implicit solvent (ε = 39.9)
surface even at elevated temperatures.42–44 This aspect corres- are illustrated. Other possible adsorbed configurations in the
ponds well to the Brønsted acid sites observed in calcined gas phase and the additional results of solution calculations
Al2O3 such as Al-500 and Al-700 given in Table 2. Wischert are given in section 5 in the ESI.† The Eads value of the
et al.44 proposed that hydroxylation over the (110) facet not adsorbed complex (at ε) was calculated using the bare surface
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Fig. 9 (a) Top view of bare γ-Al2O3 (110) and (b) top view of hydroxylated γ-Al2O3 (110) (θ = 12.4 OH per nm2). The orange ball represents O of dis-
sociated water. Structures of the isolated molecule (left), the adsorbed structure in the gas phase (middle) and the adsorbed structure in solutions
with ε = 39.9 (right) of (c) D-xylose (d) D-xylulose and (e) D-lactic acid on the catalyst .
and isolated molecule (at the same ε) as references. The calcu- (at ε = 39.9) and the C3–C4 bond is lengthened by approxi-
lated Eads values in gas and solution are summarized in mately 0.02 Å compared to the isolated gas. The C2–C3 bond
Table S2 in the ESI.† For D-xylose adsorption, the chemisorp- tends to be more activated with a bond length of 1.59 Å, when
tion occurs via the hydroxyl oxygen (O4) and the Lewis acid site D-xylose interacts via its carbonyl group (CvO) (see Config. 4
(Al4a*), as shown in Fig. 9c. Hydrogen bonding between the in Fig. S7 in the ESI†). This structure reveals less energetic
hydroxyl groups (OH) of D-xylose and the surface also strength- stability than Config. 1 in Fig. 9c.
ens the adsorption. D-xylose adsorbs on the surface with an
In Fig. 9d, D-xylulose preferably adsorbs at Al4a* via O1 of its
Eads of −1.43 eV under vacuum. Compared with the isolated OH group. The Eads values are approximately −1.34 eV and
D-xylose gas, the C–C bonds of D-xylose were lengthened during −1.52 eV in gas and solution (at ε = 39.9), respectively.
the adsorption. Under solvation, the Eads decreases to −2.03 eV Interestingly, the C3–C4 bond is activated (or weakening)
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during adsorption. In solution, the H1 of D-xylulose is sub- abundance of acid sites that play important roles in this
tracted to form water with the OH of the surface. The O1–Al4a* heterogeneous catalyst. The calculations suggested that the
length decreases from 2.04 Å in the gas phase to 1.97 Å in solu- Lewis acid (Al) sites on the γ-Al2O3 surface were reactive for
tion. Fig. 9e shows the most stable structure of D-lactic adsorp- adsorbing/activating D-xylose and D-xylulose, which is the
tion. D-Lactic acid molecularly binds with Al4a* through the O2 crucial step of the retro-aldol reaction pathway. On the other
of its carbonyl group (CvO*). The Eads values for the most hand, the production of furfural, a side-product in this reac-
stable structure in gas and solution are −1.23 eV and −1.01 eV, tion, was promoted in the catalyst-free and less Lewis acidic
respectively. The increment of Eads relates to the longer O2– catalysts (i.e. Al-1100 and ZSM-5). The solvent also plays a role
Al4a* length under the solution conditions. This aspect in the stabilities of the reactants, products and intermediates
suggested that the solvent would facilitate the desorption of as well as their interactions with the catalyst surface.
the product. The dissociative adsorption of D-lactic, that its H1 Furthermore, this reactive alumina catalyst can be fully
transfers to the catalyst surface, reveals less energetically favor- reinstalled to the original activity by simple air calcination at
able than molecular adsorption (see Fig. S7l and S7m in the 500 °C. This abundant, low-cost, non-toxic, and environmen-
ESI†). As shown in Table S2 in the ESI,† the Eads value tally friendly catalystis is one of the potential candidates for
increases when ε increases from 39.9 to 78.4. This aspect the future industrial scale production.
relates to the change of thermodynamic stability between the
adsorption state and the reference state (i.e. the isolate mole-
cule and the bare surface in solution).
Conflicts of interest
To conclude, our DFT results suggest the following
remarks. First, under the catalyst-free conditions, dehydration
of D-xylose is predominant in solution due to its thermo-
dynamic stability. Dehydration of D-xylulose is less favorable
than the direct dehydration of D-xylose via deoxy-xylose.
Second, a catalyst is necessary for lactic acid production. In
γ-Al2O3, D-xylose and D-xylulose form chemical bonds with the
Lewis acid site assisted by H-bonds with the surrounding
hydroxyl groups on the surface. These interactions weaken the
C–C bond of the adsorbed molecules and facilitate the lactic
acid production pathway. The solvent also shows a great influ-
ence on the stability of the reactants and products on the
surface. Third, D-xylulose would be one of the key intermedi-
ates for the lactic acid production pathway. In the literature, a
Lewis acid was used to isomerize xylose into xylulose.61
Therefore, the Lewis acid active sites on the Al2O3 surface
would play a decisive role in both xylose isomerisation and C–
C activation for lactic acid production. Finally, these men-
tioned aspects agree well with the observations made in our
experiment that a high amount of Lewis acid sites in Al-RT
showed beneficial effects on lactic acid production compared
to calcined Al2O3 and zeolite. Due to the complication of this
reaction, catalyst surface- and solvent-assisted mechanisms
would occur in reality. Different facets and different coverage
of hydroxyl groups on the alumina surface would be con-
sidered too. Theoretical mechanistic studies and explicit
solvent model investigations need to be further performed in
future works.
There are no conflicts to declare.
Acknowledgements
The financial support from the National Nanotechnology
Center and the SPAIII–Integrated Platform: Bio-based
Materials (P1850012) is acknowledged. We also acknowledge
the NSTDA Supercomputer Center (ThaiSC) and Nanoscale
Simulation Laboratory at National Nanotechnology Center
(NANOTEC) for computation resources.
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