Transformation of cellulose and related carbohydrates into lactic acid with bifunctional Al(III)-Sn(II) catalysts

Article abstract of DOI:10.1039/c7gc02975f

The catalytic transformation of cellulose into valuable chemicals such as lactic acid under mild conditions represents a promising route for the efficient utilization of renewable biomass. Here, we report that the combination of Al(iii) and Sn(ii) cations can efficiently catalyse the conversion of cellulose and related carbohydrates into lactic acid in water. Al(iii)-Sn(ii) is the most efficient combination for lactic acid formation among the many dual cations investigated. Al(iii) and Sn(ii) with a molar ratio of 1/1 work cooperatively, providing lactic acid with yields of 90%, 81% and 65% in the conversions of fructose, glucose and cellulose, respectively. The formation of lactic acid involves a series of tandem steps including the hydrolysis of cellulose to glucose, the isomerisation of glucose to fructose, the retro-aldol fragmentation of fructose to C₃ intermediates and the subsequent conversion of the C₃ intermediates to lactic acid. Our experimental and computational studies suggest that Al(iii) mainly catalyses the isomerisation of glucose or the C₃ intermediates, whereas Sn(ii) is primarily responsible for the retro-aldol fragmentation. The combination of the two cations enables the reaction to proceed smoothly with few side reactions, providing outstanding catalytic performances for lactic acid production from cellulose or the related carbohydrates.

Full text of DOI:10.1039/c7gc02975f

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DOI: 10.1039/C7GC02975F
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Transformation of cellulose and related carbohydrates into lactic
acid with bifunctional Al(III)-Sn(II) catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Weiping Deng, Pan Wang, Binju Wang, Yanliang Wang, Longfei Yan, Yanyun Li, Qinghong Zhang,*
Zexing Cao and Ye Wang*
DOI: 10.1039/x0xx00000x
Catalytic transformation of cellulose into valuable chemicals such as lactic acid under mild conditions represents a
promising route for efficient utilization of renewable biomass. Here, we report that the combination of Al(III) and Sn(II)
cations can efficiently catalyse the conversion of cellulose and related carbohydrates into lactic acid in water. The
Al(III)−Sn(II) is the most efficient combination for lactic acid formation among many dual cations investigated. Al(III) and
Sn(II) with a molar ratio of 1/1 work cooperatively, providing lactic acid with yields of 90%, 81% and 65% in the
conversions of fructose, glucose and cellulose, respectively. The formation of lactic acid involves a series of tandem steps
including the hydrolysis of cellulose to glucose, the isomerisation of glucose to fructose, the retro-aldol fragmentation of
fructose to C3 intermediates and the subsequent conversion of the C3 intermediates to lactic acid. Our experimental and
computational studies suggest that Al(III) mainly catalyses the isomerisation of glucose or the C3 intermediates, whereas
Sn(II) is primarily responsible for the retro-aldol fragmentation. The combination of the two cations enables the reaction to
proceed smoothly with few side reactions, providing outstanding catalytic performances for lactic acid production from
cellulose or the related carbohydrates.
www.rsc.org/
acids, which is in good agreement with the principle of high
atomic economy. Moreover, the formation of organic acids
usually occurs under non-reductive conditions and does not
1. Introduction
The catalytic transformation of renewable biomass resources,
in particular the most abundant and non-edible lignocellulosic
biomass, into chemicals, fuels and materials is the key to
establishing a sustainable chemical society.1-4 Cellulose is the
major component of lignocellulosic biomass, and thus the
efficient transformation of cellulose under mild conditions
plays an essential role in biomass valorisation.5 However, the
direct transformation of cellulose under mild conditions is
difficult due to the robust crystalline structure of cellulose.
Furthermore, various C-C and C-O bonds with similar bond
energies exist in cellulose molecules, making the selective
cleavage of specific chemical bonds to obtain a particular
valuable chemical highly challenging.
consume expensive hydrogen.
Lactic acid, which is one of the most versatile bio-based
chemicals, has found a wide range of applications in food,
pharmaceutical, cosmetic and chemical industries.15,19 As a key
building block, lactic acid can be facilely transformed into
other important chemicals in chemical industry such as acrylic
acid, pyruvic acid, propylene glycol, 2,3-pentanedione and
acetaldehyde.15,19 Furthermore, lactic acid is the monomer of
poly(lactic acid), a bio-degradable plastic and a potential
candidate to replace petroleum-derived polymers.
Currently, lactic acid is primarily produced through the
conventional biotechnological process via fermentation of
carbohydrates (usually glucose and sucrose).15 Such
a
In the past decades, tremendous endeavours have been
devoted to developing efficient catalytic systems for the
conversion of cellulose or the related carbohydrates into
useful platform compounds such as 5-hydroxymetyl furfural
(HMF),6,7 polyols,8-12 and organic acids.13-18 Among these
transformations, the synthesis of value-added organic acids
has received growing interests,13-18 because the majority of
elements such as carbon and oxygen can be maintained in the
fermentation process suffers from high enzyme cost, low
space-time yield, undesirable waste effluents and large
complexity in purification. Moreover, the fermentation is
restricted to the transformation of hexose sugars or easily
hydrolysable poly- and disaccharides from starch. In contrast,
the chemical catalysis possesses advantages of easiness in
catalyst design and reaction condition engineering to optimize
the performance. Moreover, the synthesis of lactic acid
directly from cellulose is also possible via chemical catalysis.
The development of efficient chemocatalytic systems for
lactic acid synthesis from cellulose or its derived carbohydrates
has attracted much attention in recent years. Many studies
reported that the use of alkalis (e.g., NaOH, KOH) as catalysts
under supercritical or subcritical conditions could break down
the C-C bonds in glucose, forming alkali lactates.20-23 However,
the alkali system usually shows low selectivity to lactate, and
State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative
Innovation Center of Chemistry for Energy Materials, National Engineering
Laboratory for Green Productions of Alcohols, Ethers and Esters, College of
Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
E-mail: zhangqh@xmu.edu.cn; wangye@xmu.edu.cn; Fax: +86-592-2183047; Tel:
+86-592-2186156
† Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
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the salts of many other acids such as formic, acetic, oxalic and How to control the selectivity of C-C cleavage is of
View Article Online
levulinic acids are also generated because of the severe fundamental significance. Here, we report our recent finding
DOI: 10.1039/C7GC02975F
reaction conditions (> 473 K). Co-catalysts have been that a combination of Al(III) and Sn(II) is highly efficient for
employed to improve the performance of alkali catalysts. catalytic transformations of cellulose and the related
Huang and co-workers reported that a polymer of imidazole carbohydrates into lactic acid in water. Both experimental and
and epichlorohydrin in combination with NaOH aqueous computational studies are carried out to understand in-depth
solution could offer a lactate yield of 63% during the the reaction mechanism and catalytic functions of Al(III) and
conversion of glucose at 373 K.24 The polymer was proposed to Sn(II). We attempt to gain insights into the precise cleavage of
function as weak Lewis acid sites to coordinate with key the C-C bond in carbohydrates to form target product with few
intermediates, thus facilitating the formation of lactate. side reactions through the present simple homogeneous
Choudhary et al. claimed that a magnesia-supported copper catalytic system.
accelerated the conversion of glucose to lactate in aqueous
alkaline solution, affording a lactate yield of 70% at 393 K.25
Barium hydroxide could also work as a catalyst for the
2. Experimental
conversion of glucose, affording a lactate yield up to 95% after
2.1. Materials and general methods. Most of the inorganic
48 h reaction, but a high ratio of Ba(OH)2 to glucose (10/1) was
chemicals used in this work, such as AlCl3, Al2(SO4)3, SnCl2, SnI2,
required.26
SnBr2, SnSO4, Zn(NO3)2, FeCl2, Mn(NO3)2, Cu(NO3)2, NiCl2 and
Lewis acid-based catalysts have also been reported for the
Co(NO3)2, were purchased from Sinopharam Co. In(NO3)3,
production of lactate from glucose or the related
Ga(NO3)3 and organic compounds such as cellulose, D-glucose,
carbohydrates.27-29 By incorporating Sn(IV) ions into the
D-fructose, glyceraldehyde, dihydroxyacetone, pyruvaldehyde
framework of zeolite beta, Taarning and co-workers succeeded
were purchased from Alfa Aesar. The obtained crystalline
in the conversion of glucose and sucrose into methyl lactate in
cellulose was typically ball-milled on a SFM-1 Desk-top
methanol.30 A methyl lactate yield of 64% was attained in the
planetary ball miller (MTI Corporation) for 48 h before use.
conversion of sucrose at 433 K for 20 h. The lactate selectivity
Fourier-transform ion cyclotron resonance (FT-ICR) mass
for Sn-beta could be further enhanced to 75% with the
spectrometry measurements were performed on a Bruker
assistance of K2CO3,31 which was supposed to neutralize the
APEX IV 7.0T FT-ICR mass spectrometer in positive ion mode
Brønsted acid sites in zeolites, thus inhibiting the Brønsted
equipped with an external electrospray ionisation (ESI) source.
acid-catalysed side reactions. Sn(IV)-grafted mesoporous silica
AlCl3 or SnCl2 was dissolved in water to obtain a concentration
modified with carbon also catalysed the transformation of
of approximately 0.15 mmol L-1. This solution was
glucose, fructose and sucrose in methanol to methyl lactate.32
electrosprayed from a capillary biased at 4400 V. The scan
Recently, a gallium-doped zeolite Y catalyst was reported to
range of m/z was from 100 to 600. An accuracy of m/z of
offer a methyl lactate yield of 58% from cellulose in
5
1×10− was achieved. Matrix-assisted laser desorption
supercritical methanol (553 K).33 The harsh reaction conditions
ionisation time of flight (MALDI-TOF) analysis was conducted
may facilitate side reactions (e.g., dehydration) by both
over a range of 1-2000 on Bruker MALDI-TOF mass
methanol and substrate. Efficient catalytic systems that are
spectrometer. 2,5-Dihydroxybenzoic acid was used as the
capable of transforming cellulose to lactic acid directly in non-
matrix.
alkaline aqueous solution are still scarce. Zirconia,34 tungstated
zirconia-alumina,35 and
niobium-modified
aluminium
2.2. Catalytic reaction. Conversions of cellulose, possible
hexose and triose intermediates and other biomasses were
performed in
hydroxide fluorides36 were disclosed to catalyse the direct
conversion of cellulose under hydrothermal conditions, but the
selectivity of lactic acid was low. Lanthanide triflates37 and
erbium-exchanged montmorillonite38 showed high yields of
lactic acid, but these systems either employed high-cost
catalysts or suffered from low catalyst stability.
a
batch-type Teflon-lined stainless-steel
autoclave. As an example for the conversion of cellulose, the
catalyst and the powdery cellulose were added to the reactor,
which had been pre-charged with deionised water. After the
introduction of N2 with a pressure of 3 MPa, the reactor was
placed in an oil bath. When the system reached the reaction
temperature (typically 463 K), the reaction was initiated by
vigorous stirring. After a fixed time, the reaction was quickly
terminated by cooling the reactor to room temperature in cold
water. The liquid products were analysed by a high-
performance liquid chromatography (HPLC, Shimazu LC-20A)
equipped with an RI detector and a Shodex SUGARSH-1011
column (8×300 mm) using a dilute H2SO4 aqueous solution as
the mobile phase. The conversion was calculated on the basis
of the molar difference between the substrate before and
after each reaction. The yields of main products, such as lactic
acid, glucose, fructose, HMF and levulinic acid, were calculated
from the percentage of carbon moles of products in the total
carbon moles of the substrate.
In addition to heterogeneous catalysis, homogeneous
catalysis has also attracted much attention as an important
tool for the transformation of biomass.39 In particular, the
soluble catalyst can interact better with the linkage targeted
for depolymerisation in cellulose. The molecular catalysis with
well-defined active sites can provide deeper mechanistic
insights. In a previous paper,40 we reported that a simple metal
cation, Pb(II), could efficiently catalyse the direct
transformation of cellulose in aqueous medium, producing
lactic acid with a yield as high as 65% at 463 K. However, the
toxicity of Pb(II) is unfavourable for its future application. Thus,
it is highly attractive to develop more environmental friendly
catalytic systems for the transformation of cellulose into lactic
acid. Furthermore, some aspects of reaction mechanism are
still unclear, limiting the rational design of highly selective
catalysts. The formation of lactic acid involves the cleavage of
one C-C bond. The cleavage of C-C bond may occur at different
positions in the skeleton of glucose, the monomer of cellulose.
2.3. Theoretical computation. All of the computations were
performed with the Gaussian 09 software package. The
geometries of the transition states, reactants and
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intermediates involved in the reaction were optimized using a
hydrated cluster in conjunction with the continuum solvation
model of SMD at the B3LYP/6-31G(d) level of theory (the
LanL2DZ-ECP basis set was employed for Al and Sn). Harmonic
frequency calculations were performed at the equilibrium
geometries to confirm first-order saddle points and local
minima on the potential energy surfaces and to estimate the
zero-point energy (ZPE) as well as the thermal and entropic
corrections at 298.15 K and 1 atm. The correlation between
the stable structures and the transition states was verified by
analysing the corresponding imaginary frequency mode as well
as by limited intrinsic reaction coordinate (IRC) calculations.
The relative energies of the B3LYP/6-31G(d)-optimized
structures were further refined by single-point calculations at
the B3LYP/6-311++G(d,p) level (the polarized LANL2DZdp ECP
basis set was employed for Al and Sn) with inclusion of solvent
effects. Natural bond orbital (NBO) analyses at the B3LYP/6-
311++G(d,p) level were performed to determine the atomic
charge populations. For estimation of the Gibbs free energy,
direct calculations in combination with frequency analysis in
solution were performed, which had been verified to be a
correct and practical approach41 especially when liquid-phase
and gas-phase structures differed significantly or when the
stationary points present in the liquid solution could not
survive in the gas phase. The B3LYP method was compared
with several other popular density functional theoretical (DFT)
methods including B3PW91, B97-D, M06, X3LYP and M06-2X
for the isomerisation of glucose to its enol form, and B3LYP
was found to be adequate for the present mechanistic study.
In addition to 298.0 K, thermal corrections were also assessed
at 463.0 K to explore the temperature effect on the Gibbs free
energy, and the temperature effect was not remarkable here.
View Article Online
DOI: 10.1039/C7GC02975F
Fig. 2 Catalytic conversion of cellulose catalysed by dual cations. Reaction conditions:
cellulose, 0.10 g (glucose unit, 0.62 mmol); dual-metal cations (ratio = 1/1), 0.10 mmol;
H2O, 20 mL; N2, 3 MPa; 463 K; 2 h.
in water medium. No lactic acid was formed without a cation.
Among all the single metal cations investigated, Al(III) was the
most efficient, providing a lactic acid yield of 41% at 463 K for
2 h (Fig. 1). Sn(II), In(III), Zn(II) and Mn(II) also catalysed the
transformation of fructose to lactic acid with moderate yields
of 25-35%. Other metal ions such as Co(II), Fe(II), Ni(II) and
Cu(II) were less efficient for lactic acid formation. Then, we
investigated different combinations of dual metal cations with
a molar ratio of 1/1 for the conversion of fructose at 463 K,
and the results for the combinations containing Al(III) are
displayed in Fig. 1. An outstanding lactic acid yield of 90% was
achieved using Al(III)-Sn(II). The Al(III)-Sn(II) combination is also
the most efficient for the conversion of fructose to lactic acid
among many other dual-cation combinations (Fig. S1, ESI†).
Subsequently, we performed conversion of ball-milled
cellulose in water using the dual-cation catalysts that were
efficient for the conversion of fructose. The Al(III)-Sn(II) also
exhibited the best performance for the formation of lactic acid
from cellulose, followed by Ga(III)-Sn(II), Al(III)-Mn(II) and
Al(III)-In(III) combinations (Fig. 2). A lactic acid yield of 65% was
attained in the conversion of cellulose at 463 K for 2 h with the
Al(III)-Sn(II) catalyst. We have checked whether the counter
anions influence catalytic performances. Almost the same yield
of lactic acid was obtained during the conversion of cellulose
at 463 K using the combination of different salts of Al(III) and
Sn(II) (Table S1, ESI†). Thus, Al(III) and Sn(II) cations are the
true catalytically active species for the formation of lactic acid.
The Al(III)-Sn(II) catalyst was further applied to the
conversion of microcrystalline cellulose and other biomasses
or carbohydrates such as starch, inulin, cellobiose and glucose.
As expected, the microcrystalline cellulose was more difficult
to be converted than the ball-milled cellulose, and we
obtained a lactic acid yield of 36% from the microcrystalline
cellulose after 2 h of reaction at 463 K (Table S2, ESI†). For
other biomasses or carbohydrates, lactic acid yields of 71% to
89% were obtained (Table S2, ESI†), which were comparable or
even higher than those obtained using the Pb(II) catalyst
reported in our previous paper under the same conditions.40 In
other words, the present dual-cation catalyst can work as a
more environmental benign and efficient homogeneous
catalyst for the conversion of various biomasses to lactic acid.
We further performed the conversions of glucose and
fructose with high concentrations (2.5-10 wt%) using the
Al(III)-Sn(II) catalyst. A lactic acid yield of ~50% could be
achieved for the conversion of either glucose or fructose with
a concentration of 2.5 wt% (Table S3, ESI†). The further
3. Results and discussion
3.1. Catalytic behaviours of single- and dual-metal cations for
conversions of cellulose and related carbohydrates to lactic acid
We first examined catalytic performances of various easily
accessible, cheap and less toxic single metal cations including
Al(III), In(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Sn(II)
for the conversion of fructose, a key cellulose-derived hexose,
Fig. 1 Catalytic conversion of fructose into lactic acid catalysed by various single cations
or dual-cation combinations. Reaction conditions: fructose, 0.56 mmol; single or dual
cations, 0.10 mmol; H2O, 20 mL; N2, 3 MPa; 463 K; 2 h. For dual cations, the molar ratio
of the two components was 1/1.
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increase in the concentration of sugars decreased the lactic
acid yield. Possibly, the relatively higher concentration of lactic
acid formed in the concentrated sugar solution may
significantly influence the pH value of the solution, thus
affecting the catalytic behaviours.
View Article Online
DOI: 10.1039/C7GC02975F
We have investigated the effect of pH on catalytic
behaviours of the Al(III)-Sn(II) catalyst. Since both Al(III) and
Sn(II) chlorides used in the present work were slightly
hydrolysed in water, the initial pH value of the reaction
solution was around 2.8. The formed Brønsted acids would
favour the hydrolysis of cellulose. We further regulated the pH
value in a range of 0.86-7.6 by using either HCl or NaOH and
examined the effect of pH value on the conversion of glucose
at 453 K. At a pH value of ≤ 2.8, glucose could be completely
converted (Table 1). However, a lower pH caused a decrease in
the selectivity of lactic acid and an increase in that of levulinic
acid. The formation of humins was also observed. This
suggests that an excess amount of Brønsted acids may
Fig. 3 Effect of concentration of Al(III)-Sn(II) cations on catalytic behaviours for the
conversion of cellulose. Reaction conditions: cellulose, 0.10 g (glucose unit, 0.62 mmol);
H2O, 20 mL; N2, 3 MPa; 463 K; 2 h. AlCl3 and SnCl2 were used. The molar ratio of
Al(III)/Sn(II) was 1/1.
cellulose to glucose is probably catalysed by such in-situ
accelerate undesirable side reactions and are unbeneficial to generated protons.9 HMF may be formed by acid-catalysed
dehydration of fructose, which probably arises from the
isomerisation of glucose. The introduction of Al(III)-Sn(II)
cations into the reaction system not only increased the
conversion of cellulose but also significantly changed the
product selectivity. The formation of lactic acid was triggered
by the presence of dual cations and the lactic acid selectivity
increased upon increasing the concentration up to ~5 mmol L-1
at the expense of the selectivities of glucose and HMF. The
catalyst concentration of ≥ 2.8 mmol L-1, accounting for ≥ 9.0
mol% of glucose units in cellulose, was needed to achieve ≥
50% selectivity of lactic acid. These results suggest that several
reactions including hydrolysis, isomerisation and dehydration
may occur under hydrothermal conditions, whereas the dual
cations not only enhance the conversion of cellulose but also
catalyse the formation of lactic acid via cleavage of the C-C
bond.
the formation of lactic acid. We further found that a higher pH
value (> 2.8) was also unfavourable. Both the glucose
conversion and the lactic acid selectivity decreased by
increasing the pH from 2.8 to 7.6. It is likely that the higher pH
value reduced the concentrations of Al(III) and Sn(II) cations by
increasing those of the corresponding hydroxide species via
hydrolysis. Therefore, a proper pH value is required not only to
avoid the side reactions such as the formation of levulinic acid
and humins at a lower pH value but also to keep a balance
between metal cations and their hydroxide species to ensure
the high efficiency for the formation of lactic acid.
3.2. Reaction pathways for the formation of lactic acid
The effect of the concentration of Al(III)-Sn(II) (Al/Sn molar
ratio = 1/1) on catalytic performance for the conversion of
cellulose at 463 K is displayed in Fig. 3. Cellulose could also be
converted in the absence of metal cations under hydrothermal
conditions, mainly producing glucose and HMF. No lactic acid
was formed. It is known that H3O+ ions can be reversibly
generated in hot water because of the increased ionisation
constant under hydrothermal conditions.9,42 The hydrolysis of
Table 1 Effect of pH value of the reaction solution on catalytic behaviours of Al(III)-Sn(II)
dual cations for the conversion of glucosea
pH
Conv.
(%)
Yield (%)
Fructose
Lactic acid
Levulinic acid
Othersb
7.2
5.0
3.0
3.6
0.86
1.8
2.8
4.0
7.6
100
100
100
99
0
0
0
0.7
9.5
5.6
31
81
52
10
26
15
4.4
2.9
1.3
90
16
a
Reaction conditions: glucose, 0.10
g (0.56 mmol); Al(III)-Sn(II) cations
[Al(III)/Sn(II) molar ratio = 1/1], 0.10 mmol; H2O, 20 mL; N2, 3 MPa; 453 K; 2 h.
AlCl3 and SnCl2 were used. b Others include HMF, glyceric acid, acetic acid and
formic acid. The formation of humins caused the low carbon balance at low pH
values (< 2.8), while at a high pH value (> 2.8), some unknown products led to the
poor carbon balance.
Fig. 4 Catalytic conversion of glucose at different temperatures. (a) In the absence of
metal cation catalyst. (b) In presence of Al(III)-Sn(II) catalyst. Reaction conditions:
glucose, 0.10 g (0.56 mmol); Al(III)-Sn(II) cations [Al(III)/Sn(II) molar ratio = 1/1], 0.10
mmol (in the presence of catalyst); H2O, 20 mL; N2, 3 MPa; 2 h. AlCl3 and SnCl2 were
used.
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View Article Online
DOI: 10.1039/C7GC02975F
Fig. 6 Effect of Al(III)/Sn(II) molar ratio on catalytic performances for the conversion of
glucose. Reaction conditions: glucose, 0.10 g (0.56 mmol); H2O, 30 mL; N2, 3 MPa; 423 K;
0.5 h. AlCl3 and SnCl2 were used. The total amount of Al(III) and Sn(II) was fixed at 0.10
mmol.
fructose, although HMF and lactic acid were also formed to
some extent. Thus, it is reasonable to speculate that Al(III)
cations are mainly responsible for the isomerisation of glucose
to fructose. Upon the addition of Sn(II) cation, the selectivity of
fructose decreased and that of lactic acid increased
significantly. The selectivity of HMF became further lower in
the presence of Sn(II). When the Sn(II)/Al(III) ratio reached 1/1,
i.e., Sn(II)/[Al(III)+Sn(II)] ratio = 0.5, the selectivity and yield of
lactic acid were the highest. A further increase in the ratio of
Sn(II)/Al(III) rather caused a drop in the selectivity of lactic acid.
At the same time, the conversion of glucose also decreased.
Single Sn(II) cation could catalyse the conversion of glucose to
lactic acid, but the selectivity was only 30%. The selectivities of
fructose and HMF were also low in the presence of Sn(II) alone,
and the formation of humins was observed.
Fig. 5 Reaction pathways for the conversion of cellulose in the presence and absence of
Al(III)-Sn(II) catalyst.
We carried out the conversion of glucose, a key
intermediate during the transformation of cellulose, at
different temperatures to gain more information about the
reaction pathway. In the blank reaction, fructose and HMF
were the major products, and HMF became the dominant
product at higher temperatures (Fig. 4a). The reaction solution
turned dark brown at higher temperatures, indicating the
formation of undesired humins. The presence of Al(III)-Sn(II)
significantly enhanced the conversion of glucose particularly at
lower temperatures. The formation of fructose was
remarkably accelerated at lower temperatures (393 and 413 K).
The increase in temperature in the presence of Al(III)-Sn(II)
catalyst resulted in the formation of lactic acid instead of HMF
in the blank reaction. The yield of lactic acid from glucose was
81% at 453 K.
To understand the catalytic function of Sn(II) deeply, we
have performed the conversion of fructose in the presence of
A detailed time-course analysis for the conversion of
glucose at 423 K with Al(III)-Sn(II) catalyst revealed that
fructose was initially formed as the primary intermediate (Fig.
S2, ESI†). Upon prolonging the reaction time, the selectivity of
fructose decreased and that of lactic acid increased
simultaneously. Glyceraldehyde was formed with a low
selectivity. It can be expected that retro-aldol fragmentation of
fructose may break the C3-C4 bond at the middle of the
molecule, yielding two C3 intermediates, which may be readily
converted to lactic acid or lactates.38,40,43,44 Therefore, we
propose reaction pathways in Fig. 5. In brief, the conversion of
cellulose to lactic acid catalysed by the Al(III)-Sn(II) involves the
following steps: the hydrolysis of cellulose to glucose, the
isomerization of glucose to fructose, the retro-aldol
fragmentation of fructose to C3 intermediates and the
subsequent isomerisation of C3 intermediates into lactic acid.
In the absence of a catalyst, the formation of HMF was mainly
observed. HMF may be formed from fructose by dehydration.
3.3. Catalytic functions of Al(III) and Sn(II)
Fig. 7 Conversion of fructose in the presence of Sn(II) cation. (a) Conversion and
product selectivity. (b) Selectivity of C3 compounds. Reaction conditions: fructose, 0.10
g (0.56 mmol); Sn(II), 0.10 mmol; H2O, 30 mL; N2, 3 MPa; 383 K. SnCl2 was used. The
selectivity of by-products, which mainly included polymeric compounds and humins,
were calculated by subtraction of the selectivity of known products with 100%.
To clarify the roles of Al(III) and Sn(II) in each step, we
investigated the effect of molar ratio of Sn(II)/Al(III) on the
conversion of glucose at 423 K. As shown in Fig. 6, single Al(III)
cation mainly catalysed the transformation of glucose to
This journal is © The Royal Society of Chemistry 20xx
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Green Chemistry
single Sn(II) at 383 K. We adopted a relatively mild
temperature to control the conversion of fructose with an
attempt to gain information on the primary products from
fructose catalysed by Sn(II). Fig. 7 shows that C3 compounds
are formed as the major products (selectivity, ~90%) at a short
reaction time or at initial reaction stage. Thus, it becomes clear
Sn(II). We speculate that Sn(II) may catalysVeiewtAhrteicle aOlndlionel
DOI: 10.1039/C7GC02975F
condensation of aldehyde, e.g., pyruvaldehyde, with other
aldehyde/alcohol intermediates,45 leading to the formation of
undesired polymeric by-products. Our time-course analyses for
Sn(II)-catalysed
conversions
of
pyruvaldehyde
and
glyceraldehyde showed the formations of glucose and fructose
besides lactic acid and triose isomers (Tables S4 and S5, ESI†),
confirming the occurrence of aldol condensation in the
presence of Sn(II) alone. As compared to Sn(II) alone, the
Al(III)-Sn(II) dual cations showed significantly higher selectivity
of lactic acid during the conversion of glyceraldehyde,
dihydroxyacetone or pyruvaldehyde, although the catalytic
performance of Al(III) was better. Therefore, it is Al(III) but not
Sn(II) that plays a dominating roles in the conversion of the C3
intermediates into lactic acid.
On the basis of the results described above, we propose
that Al(III) mainly catalyses the isomerisation of glucose to
fructose, while Sn(II) functions as the major active site for the
retro-aldol fragmentation of fructose to form C3 intermediates.
The conversion of C3 intermediates to lactic acid is mainly
catalysed by Al(III), whereas Sn(II) led to the polymerisation of
C3 intermediates into humins. The two cations in the catalytic
system work cooperatively for the conversion of glucose or
other carbohydrates into lactic acid.
that Sn(II) catalyses the cleavage of C3-C4 bond in fructose to
form C3 compounds. The C3 intermediates include
pyruvaldehyde, dihydroxyacetone and glyceraldehyde (Fig. 7b).
The prolonging of reaction time decreased the selectivity of
these C3 compounds, but the selectivity of lactic acid did not
increase significantly at the same time in the presence of Sn(II)
alone. Instead, polymeric by-products were formed. It has
been reported that pyruvaldehyde can be facilely polymerized
under mild conditions because of its reactive carbonyl
groups.45 Our MALDI-TOF analyses of the solid residue after
the reaction suggest the polymerization of reactive C3
aldehydes during our reactions (Fig. S3, ESI†).
We studied the conversions of three possible C3
intermediates, glyceraldehyde (GLY), dihydroxyacetone (DHA)
and pyruvaldehyde (PAD), to gain insights into the active
species for the conversion of these C3 intermediates. Table 2
displays that the three trioses can easily undergo
intermolecular transformations into each other in water even
without a metal cation catalyst. Glyceraldehyde is more active
to be transformed in the absence of a catalyst, and this can
explain our result that pyruvaldehyde and dihydroxyacetone
have been observed as the major products during the
conversion of fructose (Fig. 7b), although glyceraldehyde
3.4. Mechanistic insights from DFT computations
We have carried out theoretical computations for the
conversion of glucose to lactic acid in water to gain deeper
insights into reaction mechanism and catalytic functions of
Al(III) and Sn(II) in each elementary step. The present
computational work employs a cluster-continuum model,
which has been verified to be practicable for chemical
reactions in aqueous solutions.46,47 Our experimental studies
have already indicated the following three main steps for the
conversion of glucose into lactic acid: (1) the isomerisation of
glucose to fructose, (2) the retro-aldol fragmentation of
fructose to two C3 intermediates and (3) the isomerisation of
the C3 intermediates into lactic acid. The detailed
computational results are summarized in the electronic
supplementary information (Figs. S4-S40, ESI†). Fig. 8 displays
the mechanism based on the DFT calculation.
should be formed as
a major product along with
dihydroxyacetone by considering the C-C cleavage via retro-
aldol fragmentation of fructose.
The presence of Al(III) significantly accelerated the
conversion of each C3 substrate, catalysing the formation of
lactic acid with almost 100% yield (Table 2). Sn(II) also
facilitated the conversion of each triose, but the selectivity of
lactic acid was low. Humins were formed in the presence of
Table
2 Catalytic performances of Al(III), Sn(II) and Al(III)-Sn(II) cations for the
conversions of C3 intermediatesa
Catalyst
Blank
Substrateb Conv.
Yield (%)
GLY DHA PAD
(%)
89
57
Lactic acid
GLY
DHA
PAD
GLY
DHA
PAD
GLY
-
24
-
11
0
51
40
-
1.2
0
13
14
24
97
98
99
45
47
48
88
83
96
For the first step, i.e., the isomerisation of glucose, the
reaction involves the shift of carbonyl group from C1 to C2
position. Generally, the intermolecular hydride shift and keto-
0
0
-
43
Al(III)
Sn(II)
100
100
100
93
1.7
0
-
-
0
0
-
37
38
-
0
5.5
-
DHA
PAD
92
71
0
0
-
0
0
-
13
3.6
-
Al(III)-Sn(II) GLY
95
DHA
PAD
89
99
2.3
a Reaction conditions: substrate, 0.10 g; metal cation (or dual cations), 0.10 mmol;
b
H2O, 30 mL; N2, 3 MPa; 413 K, 0.5 h. AlCl3 and SnCl2 were used. GLY, DHA and
PAD denote glyceraldehyde, dihydroxyacetone and pyruvaldehyde, respectively.
Fig. 8 Reaction mechanism based on the DFT computations for the conversion of
glucose to lactic acid in water.
6 | Green Chem., 2017, 00, 1-3
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ARTICLE
enol tautomerisation are two possible reaction mechanisms. In computational results revealed that the energy barrier could
View Article Online
DOI: 10.1039/C7GC02975F
the alkali-catalysed isomerisation, the reaction has been be reduced in the presence of Al(III)-OH or Sn(II)-OH species
proposed to proceed through a series of enolate intermediates (Table 3). Moreover, as compared to the Sn(II)-OH, the Al(III)-
produced after the deprotonation of the α-carbonyl carbon, OH exhibited lower activation Gibbs energy, in consistent with
indicating the keto-enol tautomerisation mechanism.48,49 the experimental result in Table 2.
However, recent studies strongly suggest the intermolecular
A detailed reaction path analysis suggests that Al(III)-OH
1,2-hydride shift mechanism for the system with a transition initially triggers the deprotonation of the hydroxyl group at C2-
1
metal cation catalyst.50,51 By using H and 13C NMR isotope- OH of glucose or triose, forming a water molecule on the Al
labelled glucose, Davis and co-workers directly observed the centre (Fig. 9a). This is an exothermic reaction and further
hydrogen transfer from C2 to C1 in a Sn-based heterogeneous leads to the C1-C2 H-shift reaction. After reprotonation of the
catalytic system.52 Our previous studies revealed that the 1,2- C1-O bond with coordinated water, glucose (or triose) is
hydride shift was more favourable for the isomerisation of transformed to fructose (or lactic acid) (Fig. 9a and Figs. S11-
glucose catalysed by homogeneous Pb(II) in aqueous S13, ESI†). Similar mechanisms were also proposed for the Sn-
solution.40 Thus, we adopted the 1,2-hydride shift route for our containing zeolite-catalysed aldose-ketose isomerization.54,55 In
computations. Moreover, our ESI-MS spectra revealed that the our study, since Al(III)-OH species may stabilise its transition
Al(III) cation tended to coordinate with two glucose molecules state in C1-C2 H-shift reaction at a lower energy stage than
in the aqueous solution (Fig. S41, ESI†). Sn(II) could combine Sn(II)-OH (Figs. S13, S28, S33, S40, ESI†), Al(III)-OH shows
with either one or two glucose molecules, but the latter better performance for the isomerisation.
situation was the major case. Consequently, our computation
models were established and optimized on the basis of these the cleavage of C-C bond and the formation of C3
results (Fig. S4-S40, ESI†). intermediates. This reaction step needs to overcome an energy
The retro-aldol fragmentation of fructose is a crucial step for
In the absence of a metal cation catalyst, the activation barrier of 32.8 kcal mol-1 without a metal-cation catalyst,
barrier, expressed by activation Gibbs energy (∆G≠), for the which is the highest energy demanding step in the non-
isomerization of glucose was calculated to be 24.1 kcal mol-1.40 catalytic transformation of glucose to lactic acid. The presence
The presence of Al(III) and Sn(II) lowered the activation Gibbs of Al(III) cation slightly lowered the energy barrier. On the
energy values to 20.9 and 19.9 kcal mol-1, respectively (Table other hand, the activation Gibbs energy drops from 32.8 to
3). This indicates that both cations are capable of enhancing 19.9 kcal mol-1 in the presence of Sn(II) species, which is
this reaction. Besides the metal ions, we have also conducted significantly lower than that in the presence of Al(III).
the computations with hydroxyl-coordinated Al-OH and Sn-OH Moreover, the activation Gibbs energy with Sn(II) was lower
species, which have been reported as possible active species than that with Pb(II) (22.4 kcal mol-1).40
for the isomerization of glucose.50,53-55 Table 3 shows that
Our computational analysis for the reaction path reveals
when Al(III)-OH and Sn(II)-OH are considered as active species that the retro-aldol fragmentation of fructose is initiated by
instead of Al(III) and Sn(II), the activation Gibbs energy values the proton transfer from the C4-OH to C2=O (Fig. 9b and Figs.
decrease to 13.0 and 16.6 kcal mol-1, respectively. The Al(III)- S35-S39, ESI†). The coordination of Sn(II) to oxygen atoms of
OH species shows the lowest activation barrier. This is in fructose increases the positive charges of H atom in C4-OH.
consistent with the experimental result that the aluminium
species plays an important role in the isomerization of glucose.
The transformation of C3 compounds resulting from the
cleavage of C-C bond of fructose via retro-aldol fragmentation
into lactic acid involves several tandem elementary steps
including dehydration, hydration and isomerisation (Fig. 8).
Among them, the isomerization via the 1,2-hydride shift
mechanism has been demonstrated to be the rate-limiting
step due to its large activation energybarrier.40 Our
Table 3 Activation Gibbs energy values for the key steps during conversion of glucose
into lactic acida
Reaction stepsb
Non-catalytic
route
Catalytic route by different species
Al(III) Al(III)-OH Sn(II) Sn(II)-OH
Glucose to fructose
Fructose to trioses
Trioses to lactic acid
24.1
32.8
32.1
20.9
30.6
-
13.0
29.0
20.0
19.9
19.9
-
16.6
21.6
24.1
a Units are in kcal mol-1. b Reaction steps are isomerisation of glucose to fructose,
retro-aldol fragmentation of fructose into trioses, and the isomerisation of trioses
to lactic acid.
Fig. 9 Proposed reaction paths based on theoretical computations for the isomerization
and retro-aldol fragmentation.
This journal is © The Royal Society of Chemistry 20xx
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This means that Sn(II) can increase the acidity of C4-OH, for the formation of lactic acid. HoweverV,iew tAhrtiecle Osnildinee
DOI: 10.1039/C7GC02975F
therefore accelerating the proton transfer and the C-C bond polymerization reactions of C3 intermediates catalysed by Sn(II)
cleavage. In addition, the hydroxyl group in Sn(II)-OH may also cause the moderate selectivity of lactic acid by Sn(II) alone in
mediate the proton transfer via water forming step (Fig. 9b). water (Figs. 1 and 6). It should be noted that several papers
However, the Sn(II) species provides a relative lower activation have reported that Sn-based catalysts were efficient for the
Gibbs energy of the transition state, thus was more favourable transformation of glucose, fructose and sucrose to methyl
for the C-C bond cleavage (Figs. S37-S39, ESI†). Similar to Pb(II), lactate in methanol media.30-32 We speculate that alcohol may
Sn(II) possesses a closed shell configuration and large ionic facilely react with aldehyde intermediates, forming
radius (118 ppm) than many other metal cations, e.g., Al(III). hemiacetals, which might help to prevent the intermediates
This may favour its coordination with oxygen atoms in fructose from side polymerisations. Therefore, the incorporation of
and may allow it to accelerate the initial proton transfer step Al(III) into the Sn(II)-catalysed system provides a promising
efficiently. Moreover, the lone electron pair (5s2) of Sn(II) may way to realise the highly selective synthesis of lactic acid in
also facilitate the proton transfer,56 contributing to its water other than alcohol medium.
outstanding performance for the cleavage of C-C bond in the
retro-aldol reaction.
Under non-catalytic conditions, the dehydration of fructose
4. Conclusions
can occur, giving rise to HMF. This reaction requires a
activation Gibbs energy of 29.1 kcal mol-1,40 which is lower
than the activation barrier (32.8 kcal mol-1) for the retro-aldol
fragmentation in the absence of a catalyst. Thus, it is
understandable that HMF is formed as a major product during
the conversions of cellulose and glucose under the
hydrothermal conditions without metal cations (Figs. 3 and 4).
The presence of a metal-cation catalyst, in particular Sn(II),
decreases the activation Gibbs energy for the retro-aldol
fragmentation (Table 3), thus catalysing the precise cleavage of
C3-C4 bond.
We have clarified that the pH of reaction solution is a key
factor. An optimum pH value (~2.8) is required for the
formation of lactic acid in the presence of Al(III) and Sn(II)
cations. The appropriate pH value can avoid side reactions, e.g.,
acid-catalysed formation of levulinic acid and humins at lower
pH values (Table 1), and may help tune the concentrations of
Al(III), Sn(II) and their hydroxides such as Al(III)-OH and Sn(II)-
OH. A higher pH value may result in higher fractions of
hydroxides, which are beneficial to the isomerisation of
glucose but unfavourable for the retro-aldol fragmentation
(Table 3). This can explain why fructose was observed with
relatively high selectivity at a pH value of 7.6 during glucose
conversion (Table 1).
As described above, we have demonstrated that the
combination of Al(III) and Sn(II) can catalyse a series of tandem
reactions for the transformation of glucose into lactic acid by
decreasing the activation energy of each step. It is noteworthy
that the selective formation of lactic acid cannot be realised
only by single Al(III) or Sn(II) catalyst. This is because of the
occurrence of undesirable side reactions. For example, the
Al(III) species in the form of Al(III)-OH is highly active for the
isomerisation of glucose to fructose, but this species shows
low efficiency for the cleavage of C-C bond of fructose. Thus,
the low energy-demanding reaction, i.e., the dehydration of
fructose to HMF, proceeds during the Al(III)-catalysed
conversion of cellulose (Fig. 6), providing a low selectivity of
lactic acid. On the other hand, the Sn(II) species cannot only
enhance the isomerisation of glucose (despite not as well as
Al(III) species) but also facilitate the retro-aldol fragmentation
of fructose. Consequently, it is expected as an efficient catalyst
The present work has demonstrated that the combination of
Al(III) and Sn(II) cations efficiently catalyses the selective
transformation of cellulose or the related carbohydrates into
lactic acid in water. Under optimized conditions, the Al(III)-
Sn(II) combination could provide lactic acid yields of 65%, 81%,
and 90% from cellulose, glucose and fructose, respectively. We
have clarified that the transformation of cellulose into lactic
acid involves a series of tandem steps including the hydrolysis
to glucose, the isomerisation of glucose to fructose, the retro-
aldol fragmentation of fructose into two C3 intermediates and
the selective conversion of C3 intermediates to lactic acid. Our
experimental and computational studies reveal that the Al(III)
species accounts for the isomerisation of glucose to fructose
via 1,2-hydride shift mechanism and the conversion of trioses
to lactic acid, where the isomerisation reaction is the rate-
limiting step. On the other hand, the Sn(II) species efficiently
catalyses the retro-aldol fragmentation of fructose into two C3
intermediates. The dehydration of fructose into HMF is a
major side reaction in the presence of Al(III) without Sn(II),
whereas the polymerisation of C3 intermediates into humins
occurs as the major side reaction in presence of Sn(II) alone.
The Al(III)-Sn(II) dual cations with a molar ratio of 1/1 can
cooperate with each other and work multi-functionally for the
selective formation of lactic acid in water medium. The
coupling of multifunctional sites, which are suitable for
different elementary steps, is a useful strategy for catalyst
design for high-selective chemical transformations that contain
complex tandem elementary steps.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21690082, 91545203 and 21473141), the
Research Fund for the Doctorial Program of Higher Education
(20130121130001), the Fundamental Research Funds for the
Central Universities (20720160029) and the Program for
Innovative Research Team in University (IRT_14R31).
Conflicts of interest
8 | Green Chem., 2017, 00, 1-3
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Kim, Green Chem., 2017, 19, 1969-1982.
There are no conflicts to declare.
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1

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