Synergetic effects of bimetals in modified beta zeolite for lactic acid synthesis from biomass-derived carbohydrates

Article abstract of DOI:10.1039/c7ra12533j

An experimental study was carried out to convert carbohydrates using bimetal modified beta zeolite to obtain a maximum yield of lactic acid. The relationship between the properties and the catalytic performance of various bimetal modified beta zeolites was evaluated. The results showed that the maximum yield of lactic acid reached 52% with more than 99% glucose conversion over Pb-Sn-beta (0.3 mmol g^-1, Pb/Sn = 4:7) at 190 °C for 2 h under ambient air pressure. To evaluate the synergetic mechanism of lead and tin, key intermediates such as fructose, dihydroxyacetone, glyceraldehyde and pyruvaldehyde were used as probe reactants that were catalyzed by Pb-beta, Sn-beta and Pb-Sn-beta. The revealed key role of lead was to promote the isomerization of glucose to fructose and the retro-aldol condensation reaction from fructose to dihydroxyacetone and glyceraldehyde; meanwhile, tin had a superior catalytic performance in the dehydration of dihydroxyacetone, the hydration of pyruvaldehyde and the isomerization of pyruvic aldehyde hydrate.

Full text of DOI:10.1039/c7ra12533j

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PAPER
Synergetic effects of bimetals in modified beta
zeolite for lactic acid synthesis from biomass-
derived carbohydrates†
Cite this: RSC Adv., 2018, 8, 8965
a
b
a
a
ac
*
Meng Xia, Wenjie Dong, Minyan Gu, Cheng Chang, Zheng Shen
a
*
and Yalei Zhang
An experimental study was carried out to convert carbohydrates using bimetal modified beta zeolite to
obtain a maximum yield of lactic acid. The relationship between the properties and the catalytic
performance of various bimetal modified beta zeolites was evaluated. The results showed that the
maximum yield of lactic acid reached 52% with more than 99% glucose conversion over Pb–Sn-beta
(0.3 mmol gꢀ1, Pb/Sn ¼ 4 : 7) at 190 ꢁC for 2 h under ambient air pressure. To evaluate the synergetic
mechanism of lead and tin, key intermediates such as fructose, dihydroxyacetone, glyceraldehyde and
pyruvaldehyde were used as probe reactants that were catalyzed by Pb-beta, Sn-beta and Pb–Sn-beta.
The revealed key role of lead was to promote the isomerization of glucose to fructose and the retro-
aldol condensation reaction from fructose to dihydroxyacetone and glyceraldehyde; meanwhile, tin had
a superior catalytic performance in the dehydration of dihydroxyacetone, the hydration of pyruvaldehyde
and the isomerization of pyruvic aldehyde hydrate.
Received 17th November 2017
Accepted 10th February 2018
DOI: 10.1039/c7ra12533j
rsc.li/rsc-advances
have several drawbacks related to complex purication, unex-
pected salt waste effluent and slow kinetics.14,15 The method of
1. Introduction
enzymatic and lactonitrile hydrolysis also confront with the
high cost of enzyme and using highly toxic hydrogen cyanide,
respectively. Based on these problems, alternative chemo-
catalytic processes to generate lactic acid or the corresponding
esters have been the subject of extensive attention.
Biomass is sustainable, nontoxic, cheap and abundant in
nature. Based on these properties, the efficient utilization of
biomass to produce liquid fuels and chemicals has attracted
signicant attention on a global scale.1–3 Among the various
synthetic chemicals, lactic acid is one of the top 12 bio-based
In the chemocatalytic process, Hayashi and Sasaki rst re-
ported that trioses were catalytically converted into alkyl
lactates in alcoholic media by homogenous catalysts in 2005.16
Since then, other starting materials such as glucose,17–19 glyc-
erol20,21 and cellulose22–24 as well as raw biomass25,26 also have
been explored to generate high yield of lactic acid or its esters in
homogenous catalytic system. Among these, Zn, Ni and acti-
vated carbon have been found to greatly promote yields of lactic
acid from cellulose under alkaline hydrothermal conditions.27
More signicant yields of lactic acid have obtained approxi-
mately 70% from conversion of milled cellulose in 20 ml of
PbIINO3 (7 mM); at the same time, theoretical and experimental
studies have suggested that lead(II) played a key role in a series
of cascading reactions for lactic acid formation.28 Subsequently,
the two-component In–Sn homogenous catalyst system has
been demonstrated to effectively synergistically catalyze several
elemental reactions for MeLac syntheses from hexoses.29 These
metal species, as homogenous catalysts, have advantages in
producing lactic acid or its esters from different materials, but
difficulties with separation and recovery from the reaction
mixtures. Heterogeneous catalysts can effectively overcome
those disadvantages, and they have been extensively researched
“platform molecules”, as
a starting point for catalytic
synthesis, which can be further catalytically converted into
high-value-added chemicals.4–7 Moreover, lactic acid is also
considered a renewable chemical building block for polylactic
acid (PLA), which has the potential to replace fossil-derived
plastics and to emerge expansive promise in markets.8 Due to
its extensive applications in pharmaceuticals, food industry,
cosmetics and biodegradable plastic, the demand for lactic acid
has increased.9 Currently, production of lactic acid mainly
includes microbial fermentation, enzymatic and hydrolysis of
lactonitrile. Among of these, approximately 70% of lactic acid is
harvested by microbial fermentation of carbohydrates in the
world,10–13 although very selective, these biological processes
aState Key Laboratory of Pollution Control and Resources Reuse, Key Laboratory of
Yangtze River Water Environment of Ministry of Education, College of
Environmental Science and Engineering, Tongji University, Shanghai 200092, China.
E-mail: zhangyalei@tongji.edu.cn; shenzheng@tongji.edu.cn
bZhejiang Scientic Research Institute of Transport, Hangzhou 311305, China
cNational Engineering Research Center of Protected Agriculture, Tongji University,
Shanghai 200092, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12533j
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in recent years.30–38 In particular, the Sn-beta zeolite has been dealuminated beta zeolite (deAl-beta). Then, the obtained deAl-
ꢁ
applied to obtain 64% methyl lactate directly from sucrose in beta zeolite was dried at 150 C overnight.
methanol at 160 ꢁC.39 However, the yield of lactic acid in
The bimetallic beta zeolites were prepared by grinding deAl-
aqueous solution was far lower than in an organic solution beta (1 g) with the corresponding acetates for 30 min. Subse-
under the same conditions. Hence, exploring an appropriate quently, the samples were calcined in an air ow at 550 ꢁC with
ꢁ
catalyst to promote the yield of lactic acid in aqueous solution a ramp of 2 C per minute for 360 min. The different lead and
remains a challenge. The conversion of sugars into lactic acid tin loading beta zeolites (0.2 mmol gꢀ1, 0.4 mmol gꢀ1, 0.8 mmol
proceeds via a series of complex cascading steps, and different gꢀ1, 1.6 mmol gꢀ1, and 3.2 mmol gꢀ1; Pb/Sn ¼ 1 : 1) were
metal species possibly improve different key steps in the cata- prepared following the method described above. These samples
lytic reaction. Based on previous research by our group,40 we were denoted as Pb–Sn-beta-1, Pb–Sn-beta-2, Pb–Sn-beta-3, Pb–
proposed a design strategy that different metal species incor- Sn-beta-4, and Pb–Sn-beta-5. The beta zeolite containing lead
porated into Sn-beta zeolites to further promote the yield and alone was denoted as Pb-beta, and the beta zeolite modied
selectivity of lactic acid from carbohydrates conversion in with tin alone was denoted as Sn-beta.
aqueous media.
In the present work, we prepared all kinds of bimetal
modied beta zeolite catalysts including different metal loading
Pb–Sn-beta as well as Ni, Cu and Ce modied Sn-beta zeolites by
solid-state ion-exchange method to convert sugars into lactic
acid in aqueous solution under mild conditions. The relation-
ship between the catalytic activity and physicochemical prop-
erties of catalysts with different loading of lead and tin in beta
zeolites was examined. The effect of reaction conditions on
lactic acid yield was evaluated. Moreover, key intermediates
such as fructose, dihydroxyacetone, glyceraldehyde and pyr-
uvaldehyde were used as probe reactants that were catalyzed by
Pb-beta, Sn-beta and Pb–Sn-beta to discuss the synergistic
mechanism of lead and tin, while the reaction route was being
proposed.
2.3 Catalyst characterization
The contents of silicon, aluminum, lead and tin in the catalysts
were determined by inductively coupled plasma optical emis-
sion spectroscopy (ICP-OES, Perkin Elmer Optima 2100 DV).
The powder X-ray diffraction (XRD) data were obtained on
a Bruker D8 Advance X-ray powder diffractometer (40 kV, 40 mA,
˚
CuKa radiation of 1.54 A) to characterize the crystal structures
over a 2q range of 10ꢁ to 70ꢁ at room temperature. The nitrogen
adsorption–desorption experiment was performed using
ꢁ
a Micromeritics ASAP2020M analyzer at ꢀ196 C. The specic
surface area (SBET) was calculated with the Brunauer–Emmett–
Teller (BET) method over a range of relative pressures from
0.005 to 0.25. The pore-size distribution was derived from the
desorption branches of the isotherms using the Barrett–Joyner–
Halenda (BJH) method and the total pore volume (Vtotal) was
estimated at a relative pressure (P/Po) of approximately 0.99. The
X-ray photoelectron spectra (XPS) of dehydrated samples were
carried out using PHI-5000C ESCA system with Al Ka radiation
(hn ¼ 1486.6 eV).
2. Materials and methods
2.1 Materials
D-(+)-Glucose (99.5%), D-(ꢀ)-fructose (99%) and tin(II) acetate
were obtained from Sigma-Aldrich. D-(+)-Mannose (99%),
lactose (98%), starch, formic acid ($98%), acetic acid (99.8%),
glycolic acid (98%), levulinic acid (99%), oxalic acid (99%),
pyruvaldehyde (40%), 1,3-dihydroxyacetone dimer (97%),
stannic oxide (99.5%), nickel(II) acetate (99%) and cerium(III)
acetate (99%) were purchased from Aladdin. Sucrose (99%),
lead (II/IV) oxide (95%), lead(II) acetate (99.5%) and copper(II)
acetate (99%) were obtained from the Sinopharm Chemical
Reagent Co. Ltd. (Shanghai, China). Cellulose (microcrystalline)
and lactic acid (1.0 M) were purchased from Alfa Aesar. 5-
Hydroxymethylfurfural (98%) was purchased from J&K Scien-
tic. A commercial beta zeolite (Si/Al ratio of 25) was purchased
from Catalyst Plant of Nankai University (Tianjin, China). All
reagents were used as-received without further purication.
The total acidity of the catalysts was obtained by NH3
temperature-programmed desorption (TPD) using a Micro-
meritics AutoChemII 2920. The 100 mg catalyst was pretreated
in helium at 300 ꢁC for 1 h, aer which the catalyst was cooled to
120 ꢁC in a helium ow. Subsequently, 5% ammonia in the
helium ow was absorbed on catalyst at 120 ꢁC for 2 h, and then
the dry helium ow was used to remove the weakly adsorbed
ammonia. Finally, NH3-TPD was performed from 120 ꢁC to
800 C at the ramp of 10 C minꢀ1 in the dry helium ow.
FT-IR spectra of the adsorbed pyridine studied the acidic
properties of samples. In the measurement, each sample (10
mg) was pressed into a self-supported wafer with a diameter of
13 mm. The wafer was placed in a quartz IR cell sealed with
CaF2 windows and connected to a vacuum system, in which the
ꢁ
ꢁ
ꢁ
samples were dried at 450 C for 2 h under vacuum and then
cooled to room temperature. Subsequently, the wafer was
exposed to pyridine vapour and allowed to adsorb pyridine for
30 min. Finally, the desorption step at 150 ꢁC (60 min) was
performed.
2.2 Catalyst preparation
The catalysts were prepared by a solid-state ion-exchange
following the published protocol.40,41 The typical procedure
was that the beta zeolite (13 g) was treated with an HNO3
solution (65 wt%, 250 ml) at 100 ꢁC for 20 h, followed by
2.4 Reaction tests and product analysis
centrifuged and repeatedly washed with deionized water until All catalytic reactions were performed in a stainless steel auto-
the supernatant was neutral (pH
¼
7) to obtain the clave with a closed Teon vessel (50 ml). In a typical experiment,
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the vessel was charged with carbohydrate (7.5 mmol C), catalyst
(200 mg) and water (10 ml), and the autoclave was heated to
190 ꢁC in a rotating oven (20 rpm). Once the set temperature was
attained, the reactor was held at that temperature for 2 h, aer
which the autoclave was quenched in a cold bath. Finally, the
reaction mixture was separated by centrifugation, while the
supernatant liquid was diluted with deionized water before
analysis.
The concentration of sugars, organic acids (i.e. lactic acid,
glycolic acid, formic acid and acetic acid) and 5-hydrox-
ymethylfurfural was analyzed on an Agilent 1200 series HPLC
(Bio-Rad HPX-87H) with a RI and UV detector (210 nm) using
a 5 mM aqueous sulphuric acid solution as the eluent at a ow
rate of 0.5 ml minꢀ1. The column and RI detector temperature
were set at 55 ꢁC and 45 ꢁC, respectively. All products were
determined based on the external standard. The conversion of
sugars and the yield of products were determined based on
carbon as follows:
Fig. 1 Comparison of different catalysts for the conversion of glucose.
Reaction conditions: 190 ꢁC, 2 h, air, 225 mg glucose and 200 mg
catalyst. LA: lactic acid, AA: acetic acid, FA: formic acid, and GLY:
ꢀ
ꢁ
moles of unconverted sugar
moles of initial sugar
Sugar conversion ðC%Þ ¼ 1 ꢀ
glycolic acid. Metal content of Sn-beta + Pb-beta: Sn ¼ 0.2 mmol gꢀ1
,
Pb ¼ 0.2 mmol gꢀ1
.
ꢂ 100%
zeolite.42 The yields of LA and HMF decreased, when deAl-beta
was used as a catalyst, especially HMF. This result occurred
due to the decreasing acidity of deAl-beta. In the case of SnO2,
Pb3O4 and SnO2 + Pb3O4, low yields of LA were generated,
indicating they do not affect lactic acid synthesis. However,
when beta zeolite containing lead and tin (Pb–Sn-beta-1) was
used, the yield of LA dramatically increased to approximately
39%, whereas the yield of HMF was only 9%. The yields of LA
initially increased and then decreased with further increases in
lead and tin loading. As the contents of lead and tin exceeded
0.8 mmol gꢀ1 (Pb–Sn-beta-3), the yields of LA sharply declined,
implying an abrupt change of catalyst properties. Moreover, the
catalytic performance of Sn-beta + Pb-beta was investigated to
clarify the effect of the metal located sites in beta zeolites on
products yields. The result indicated that located sites of lead
and tin have no effect on LA yield, because Sn-beta + Pb-beta
(0.4 mmol gꢀ1) and Pb–Sn-beta-2 (0.4 mmol gꢀ1) have compa-
rable LA yield. The maximum yield of LA was up to 45% within
the loading scope of lead and tin of 0.2–0.4 mmol gꢀ1. It was
well accepted that Brønsted acid sites can catalyze the hydro-
lysis of monosaccharide,43 and Lewis acid sites can catalyze the
isomerization of glucose to fructose, followed by retro-aldol
condensation and a series of isomerization reactions to lactic
acid.44 Based on this theory, we speculate that appropriate
loading of lead and tin tunes the acidity of the beta zeolite,
generating great potential for glucose conversion and lactic acid
production. To conrm the hypothesis, the physicochemical
properties, structures and acidity of beta zeolites with different
lead and tin contents were explored.
moles of carbon in phase product
moles of carbon in initial sugar
Product yield ðC%Þ ¼
ꢂ 100%
3. Results and discussion
3.1 Catalytic performance of different bimetal modied beta
zeolites
The transition metals (Ni and Cu), lanthanide (Ce) and primary
group element (Pb) were selected to modify Sn-beta zeolite
catalysts for conversion glucose into lactic acid. The results
indicated that Sn-beta zeolite catalysts containing different
metal species exhibited signicant difference catalytic activities
for production of lactic acid (Table S1†). The median LA and
high HMF yield together with a small amount of formic acid,
acetic acid and glycolic acid were generated using Ni–Sn-beta,
Cu–Sn-beta or Ce–Sn-beta catalyst. The high LA and low HMF
yields were obtained over Pb–Sn-beta. The conversions were all
greater than 99% for all catalysts. Hence, Pb–Sn-beta zeolite
catalyst was selected to further evaluate the catalytic activity of
conversion glucose to lactic acid.
3.2 Catalytic performance of lead and tin modied catalysts
for glucose conversion
The results of glucose conversion into lactic acid using different
catalysts are shown in Fig. 1. A negligible yield of LA was ob-
tained without adding catalyst. In the presence of the beta, the
yield of LA was low, approximately 6%, and the main product
was HMF with a yield of 15%, together with a small amount of
3.3 Textural properties and XRD
formic acid, acetic acid and glycolic acid. The result can be The results of the textural properties determined by nitrogen
explained to internal Brønsted and Lewis acids of the beta physisorption were showed in Table 1. The raw beta zeolite had
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Table 1 Physicochemical properties of catalysts
Catalyst
SBET (m2 gꢀ1
)
Vtotal (ml gꢀ1
)
Vmicropore (ml gꢀ1
)
Pba (mmol gꢀ1
)
Sna (mmol gꢀ1
)
Si/Ala ratio
Beta
deAl-beta
585.0
578.5
576.0
573.7
533.0
441.4
323.9
574.0
577.4
0.345
0.365
0.358
0.351
0.320
0.293
0.205
0.362
0.402
0.188
0.189
0.183
0.179
0.167
0.137
0.103
0.189
0.184
—
—
—
—
25
>1700
>1700
>1700
>1700
>1700
>1700
>1700
>1700
Pb–Sn-beta-1
Pb–Sn-beta-2
Pb–Sn-beta-3
Pb–Sn-beta-4
Pb–Sn-beta-5
Pb–Sn-beta1thb
Pb–Sn-beta2thc
0.08
0.19
0.38
0.81
1.62
0.15
0.09
0.11
0.21
0.41
0.79
1.61
0.16
0.15
a
b
Determined by ICP-OES analysis. Catalyst aer rst calcination. c Catalyst aer second calcination.
a BET surface area of 585.0 m2 gꢀ1, total pore volume of be used to inspect the change of unit cell volume induced by
0.345 ml gꢀ1 and micropore volume of 0.188 ml gꢀ1. Deal- incorporation of lead and tin species. Using calculated d302 (2q
umination of beta zeolite decreased the BET surface area (578.5 ¼ 22.4–22.5ꢁ) of raw beta and deAl-beta suggested that there was
m2 gꢀ1) and increased total pore volume (0.365 ml gꢀ1). The a contraction (ꢃ3.977–3.930 A) in the unit cell volume of deAl-
˚
reason was that dealumination resulted in mesopore genera- beta zeolite. With the lead and tin incorporation into deAl-
tion and increase the average pore diameter. With the lead and beta zeolite, the unit cell volume had a slight re-expansion
˚
˚
tin incorporation into beta zeolites (Pb–Sn-beta-1, Pb–Sn-beta-2, (3.972 A for Pb–Sn-beta-1 and Pb–Sn-beta-2, 3.956 A for Pb–
Pb–Sn-beta-3), the BET surface areas and pores volumes Sn-beta-3); however, no obvious changes were observed in the
decreased. The observed decrease of BET surface areas and typical diffraction pattern. This result indicated that vacant
accessibility of the pores may be caused by occupation of framework sites were occupied upon incorporation of lead and
channels or opening pores from metal species. As contents of tin species. In addition, there was a complete absence of any
lead and tin species exceeded 0.8 mmol gꢀ1 (Pb–Sn-beta-3) in SnO2 and Pb3O4 reections, suggesting that either other
beta zeolite, the BET surface areas, total pore volumes and amorphous metal oxide (SnxOy and PbxOy) existed or SnO2 and
micropore volumes evidently decreased from 533.0 to 323.9 m2 Pb3O4 highly dispersed in the zeolites pores. The results were in
gꢀ1, 0.320 to 0.205 ml gꢀ1 and 0.167 to 0.103 ml gꢀ1, respec- accordance with previous studies, which reported metal oxides
tively. This was ascribed to the pores blocked from lead and tin might be non-crystalline phase or good dispersion when the
over-loading, which corresponding to the inferior catalytic metal content was low.41,46,47 However, with metal species
activity for glucose conversion into lactic acid.
contents beyond 0.8 mmol gꢀ1 in beta zeolite (Pb–Sn-beta-3),
X-ray diffraction patterns of the samples are shown in Fig. 2. further increased contents of lead and tin drastically
Compared to the raw beta zeolite, dealumination with HNO3 decreased intensities of the typical peak (14.5ꢁ, 22.5ꢁ and 30.0ꢁ),
did not affect the phase structure, which suggests that the which may be ascribed to partial loss of crystallinity resulting
framework of deAl-beta remained fully intact. Besides, deAl- from metal species over-loading. Moreover, the SnO2 and Pb3O4
beta zeolite possessed a high ratio of Si/Al (>1700) introducing produced a very clear diffraction peaks (26.3ꢁ, 26.7ꢁ, 33.9ꢁ, 52.0ꢁ
an amount of vacant framework sites, which would be able to be and 51.8ꢁ) in beta zeolites, indicating that extraframework
occupied by incorporated metal species.41,45 The d-spacing can metal oxides generated with metal species contents beyond
0.8 mmol gꢀ1 during calcinations. And the peaks intensities of
corresponding oxides increased with increasing metal species
content, which were all corresponding to the inferior catalytic
activity. The reason was that SnO2 and Pb3O4 had a negligible
catalytic activity for glucose conversion into lactic acid, and
amounts of metal oxides covered the catalytic activity sites.
3.4 States of Sn and Pb in beta zeolites
The XPS curves of tin and lead are showed in Fig. 3. The binding
energies (BEs) of the elements were calibrated with respect to
the C 1s peak at 284.6 eV. The Sn 3d XPS spectra exhibited two
distinct Sn 3d5/2 and Sn 3d3/2 states at 487.7 and 496.2 eV,
respectively in different catalysts, which would be attributed to
tetrahedrally coordinated framework Sn species. However, the
3d5/2 and 3d3/2 binding energy values of extra-framework Sn
species positioned at 486.0 and 494.4 eV, which is characteristic
Fig. 2 X-ray diffraction patterns of various catalysts.
8968 | RSC Adv., 2018, 8, 8965–8975
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Fig. 3 XPS spectra of the Sn 3d and Pb 4f states of different catalysts.
that of Bo Tang et al.,50 while also indicated that tin was and tin into the framework of the deAl-beta zeolite. With metal
incorporated into the framework of catalyst. In the Pb 4f region, species loading below 0.8 mmol gꢀ1 (Pb–Sn-beta-3), the samples
the signals appeared at binding energy for Pb 4f7/2 of 139.9 eV, presented two desorption peaks at approximately 160 ꢁC and
which was greater than the binding energy of pure Pb3O4 (137.9 600 ꢁC that, respectively, corresponding to weak and strong acid
eV), indicating that lead did not exist as extra-framework Pb sites. With increased lead and tin loads, the strong acid sites
species. The XPS results revealed that tin and lead species were gradually decreased, whereas the weak acid sites increased.
coordinated in BEA framework with low metal loading (below However, as metal species loading exceeded 0.8 mmol gꢀ1, the
0.8 mmol gꢀ1). The results were accordance with the results samples were dominated by weak acid sites.
from the nitrogen physisorption and XRD.
To distinguish Lewis and Brønsted acid sites, the IR spectra
of various catalysts adsorbing pyridine are shown in Fig. 5. The
peaks at approximately 1449 cmꢀ1 and 1540 cmꢀ1 were assigned
to Lewis acid sites and Brønsted acid sites, respectively. The
peak at 1490 cmꢀ1 was attributed to pyridine absorbed on both
Lewis and Brønsted acid sites (B + L). Following the raw beta
zeolite was treated with HNO3, the peak at 1540 cmꢀ1 and
1490 cmꢀ1 disappeared, and the peak at 1449 cmꢀ1 decreased in
intensity. As lead and tin were incorporated into the deAl-beta
zeolite, the Brønsted acid sites were still absent, while the
Lewis acid sites gradually increased with increasing metal
3.5 Acidity
The NH3-TPD pattern of each catalyst is depicted in Fig. 4. The
large and medium desorption peak of raw beta zeolite centered
ꢁ
ꢁ
at approximately 200 C and 320 C indicated that it had weak
and medium acid sites. In the case of deAl-beta zeolite, the weak
and medium acidic site accordingly disappeared following the
removal of Al from the raw beta zeolite. However, the surface
acidity was signicantly changed with the incorporation of lead
Fig. 4 NH3-TPD curves of various catalysts: (a) beta; (b) deAl-beta; (c) Fig. 5 FTIR spectra following the adsorption of pyridine on various
Pb–Sn-beta-1; (d) Pb–Sn-beta-2; (e) Pb–Sn-beta-3; (f) Pb–Sn-beta- catalysts: (a) beta; (b) deAl-beta; (c) Pb–Sn-beta-1; (d) Pb–Sn-beta-2;
4; and (g) Pb–Sn-beta-5.
(e) Pb–Sn-beta-3; (f) Pb–Sn-beta-4; (g) Pb–Sn-beta-5.
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species loading (curves c–f). Although the Pb–Sn-beta-3 and Pb– in Table S3.† The monosaccharides were completely converted,
Sn-beta-4 had more Lewis acid sites than Pb–Sn-beta-2, the and the lactic acid yields showed no signicant difference, as
catalytic performance of Pb–Sn-beta-2 was superior to that of they all obtained yields greater than 50%. The disaccharides
both. Coincidentally, research has indicated that catalyst with were also completely converted, but the LA yield was much
excess Lewis acidity form humins from glucose.47 We also found different. The sucrose, consisting of one fructose and one
that higher Lewis acid sites increased the yield of humins, while glucose unit, and the lactose, consisting of one glucose and one
the LA yield was lower. Therefore, the catalyst with appropriate galactose unit, yielded 51.2% and 37.8% LA, respectively. The
Lewis and Brønsted acid sites seemed suitable for lactic acid results suggested that catalytic conversion of sucrose and
production. The contents of Lewis and Brønsted acid sites of lactose may proceed via a different reaction route. When starch
different catalysts are shown in Table S2.†
and cellulose were used as feedstock, which are polysaccharides
consisting of a large number of glucose units, conversion of
both were approximately 30%, and lactic acid yields decreased
to 10.1% and 16.2%, respectively. These results indicated that
depolymerization of polysaccharides might need higher
temperature and longer reaction time than present reaction
condition (190 ꢁC and 2 h) or that Pb–Sn-beta catalyst might not
have catalytic activity for conversion cellulose and starch into
lactic acid. In the future work, we will change the reaction
condition or catalyst to improve the yield of lactic acid from
polysaccharides.
3.6 Catalyst reusability
To make uttermost use of catalysts, the reusability of the Pb–Sn-
beta zeolite in the conversion glucose into lactic acid was
investigated. First, catalysts were recovered by centrifugation
and separation aer the rst trial, dried at 70 ꢁC overnight, and
ꢁ
then calcined at 550 C to run again under identical reaction
conditions. The catalytic activity results of reusable catalysts are
presented in Fig. S1.† The glucose conversion decreased from
99% to 87% and LA yield dropped from an initial value of 52%
to 33% over the course of three recycling trials. In contrast, the
yield of HMF increased, indicating that the catalytic character-
istics of Pb–Sn-beta might be changed in the process of recy-
cling trials. To further explore the reasons for catalyst
deactivation, the used catalyst was characterized by XRD
(Fig. S2†), nitrogen physisorption analysis (Table 1) and pyri-
dine FT-IR (Table S2†). The results of the XRD pattern analysis
revealed that used Pb–Sn-beta did not have difference in phase
characteristics, compared with the fresh catalyst. The nitrogen
physisorption analysis showed that SBET, Vmicro and Vtotal of the
second calcined catalyst increased to 577.4 m2 gꢀ1, 0.184 ml gꢀ1
and 0.402 ml gꢀ1, respectively, compared with fresh catalyst.
This could be ascribed to leaching of metal species and
formation of a great number of mesoscopic pores. Moreover,
pyridine FTIR analysis indicated that Lewis acid sites gradually
decreased, while Brønsted acid sites were almost unchanged
with used times. Lewis acidity was decreased because metal
species were leached. The result was also conrmed by ICP-OES
analysis (Table 1), which showed that the contents of lead and
tin decreased to 0.09 mmol gꢀ1 and 0.15 mmol gꢀ1 aer the
second calcination. The changes in Lewis and Brønsted acid
sites obviously affected the products distribution. Due to rela-
tively increased Brønsted to Lewis acid sites ratio, the dehy-
dration step of monosaccharide to HMF was promoted, and the
conversion step of intermediates from monosaccharide disso-
ciation to lactic acid was inhibited. Hence, the observed
decrease in catalytic performance during recycling mainly could
be due to the leaching of metal species. This indicated that the
stability of the Pb–Sn-beta catalyst prepared by the solid-state
ion-exchange method still needs to be improved.
3.8 Effect of reaction conditions on the conversion of
glucose over Pb–Sn-beta
To achieve the maximize LA yield, a series of reaction conditions
were evaluated. The Fig. S3† depicts the effect of different
catalyst dosages on products yields and the conversion of
glucose. The LA yield increased before the catalyst dosage
reached 200 mg and the mass ratio of catalyst to glucose was
0.9. As the catalyst dosage exceeded 200 mg, the LA yield was
almost invariable and reached a plateau at approximately 50%.
Conversely, HMF yield consistently decreased with increasing
catalyst dosage. The yield of other products was nearly
unchanged in the range of catalyst dosage from 40 to 280 mg.
When the catalyst dosage was chosen to be 200 mg, the products
yields as a function of reaction temperature are described in
Fig. S4.† The LA yields increased sharply as the reaction
temperature increased from 150 ꢁC to 190 ꢁC, and they slightly
declined beyond 190 ꢁC. The yields of other products (HMF, FA,
and GLY) also presented the same trend. The highest LA yield of
52% was obtained when the reaction time was 2 h at 190 ꢁC, as
shown in Fig. S5.† The LA yield varied little when the reaction
time exceeded 2 h. In contrast, HMF yield gradually decreased
with increased reaction times. The results suggested that HMF
decomposed to other products at longer reaction times. The
yield of other products, such as FA, GLY, AA, and Ox, varied
slightly at prolonged reaction times.
Atmosphere condition also dramatically inuenced product
yields. Three kinds of atmosphere condition (air, nitrogen and
oxygen) under ambient pressure were investigated (Fig. S6†). As
the catalyst dosage, temperature and time were xed, the yields
of LA were largest in air, followed by in N2 and then, in O2. The
3.7 Conversion of other biomass over Pb–Sn-beta
The various biomass materials, including monosaccharides yield of HMF exhibited also similar order. The highest yields of
(fructose, glucose and mannose), disaccharides (sucrose and LA were achieved in ambient pressure air, which was conve-
lactose) and polysaccharides (starch and cellulose), were nient, safety and lower energy consumption to producing lactic
hydrothermally converted over the Pb–Sn-beta zeolite, as shown acid in industrial process.
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The effects of the lead to tin molar ratio on the yields of and tin. Subsequently, Lewis acids catalysts facilitate the retro-
products are shown in Fig. 6. The conversion of glucose was aldol condensation of fructose to produce two C3 intermediates
higher than 98% in all cases. As the Pb/Sn molar ratio was (that is glyceraldehyde and dihydroxyacetone), and then glyc-
approximately from 3/7 to 4/6, the maximum LA yield was eraldehyde and dihydroxyacetone may undergo dehydration to
generated. The LA yield gradually decreased with Pb/Sn molar 2-hydroxypropenal that produce pyruvaldehyde via keto–enol
ratios beyond 4/6. The HMF yield was nearly invariable for tautomerization. Finally the pyruvaldehyde isomerizes into
different lead to tin molar ratios (3/7–7/3). However, HMF had lactic acid by Lewis acid facilitating the nucleophilic attack of
the highest yield over beta zeolite containing tin alone (Sn-beta) the electron donor to the carbonyl functional group of the
and lowest yield over beta zeolite containing lead alone (Pb- aldehyde, thereby completing intramolecular rearrangement by
beta). Other main products including FA, AA, GLY had a rela- the shi of hydride. For this complex cascade reaction, many
tively lower yield compared with LA yield. Apparently, the lead studies have demonstrated that Sn-beta plays a key role as
to tin molar ratio had a pronounced effect on the products a Lewis acid; in addition, Wang et al. applied theoretical
distribution. The yield of LA was closed to 52% together with calculations to show the positive effect of lead(II) species on this
6.4% HMF, 2.8% AA, 4.8% FA and 1.2% GLY at the lead to tin process.28,31,44 Therefore, we speculate that lead and tin species
molar ratio of 4/7. However, when the lead to tin molar ratio was cooperatively interact in this complex cascade.
0/10 or 10/0, the LA yield was 26.8% or 26.5%, respectively,
To verify the above hypothesis regarding the reaction
which was far lower than the 52% LA yield. These results indi- pathway and to explore synergistic effects of lead and tin in the
cated that lead and tin incorporated into beta zeolite had catalytic reaction, the key intermediates (fructose, dihydroxy-
a strong synergistic effect on the conversion of glucose to lactic acetone, glyceraldehyde and pyruvaldehyde) were used as the
acid. The ICP results of different Pb/Sn molar ratio are shown in probe reactants that were catalyzed by Pb-beta, Sn-beta, and Pb–
Table S4.†
Sn-beta zeolites, respectively. The results with glucose, fructose,
dihydroxyacetone, glyceraldehyde, and pyruvaldehyde as
a function of time are shown in Fig. S7.† In the reaction of
glucose, the reaction rate was much faster, and the LA yield was
higher over Pb–Sn-beta zeolite than over the Sn-beta or Pb-beta
zeolites. The yield of LA was 26.5%, 22.4% and 52% over Pb-
beta, Sn-beta and Pb–Sn-beta zeolites, respectively, for
120 min. A similar trend was found when using fructose as the
feedstock. The yield of LA was 20%, 40% and 55% using Pb-
3.9 Reaction mechanism and catalytic synergistic function
of lead and tin
It was well accepted that Lewis acids can promote the isomeri-
zation of glucose to fructose.51,52 In this process, many studies
have proposed that the hydrolyzed framework tin species in Sn-
beta zeolite were responsible for the isomerization of glucose to
fructose; moreover, 13C NMR and H NMR studies conrmed
1
the isomerization process by 1, 2-H shi.51 In addition, Pb(II)–
OH species in aqueous also demonstrated the key role for
isomerization of glucose to fructose.28 We speculate that the
reaction pathway for the conversion of glucose into lactic acid
over the Pb–Sn-beta zeolite begins with isomerization of glucose
to fructose via 1, 2-H shi by a cooperative interaction with lead
Table 2 Conversion of different carbohydrates as the probe reactants
over different beta zeolitesa
The yield of aqueous-phase
products (%)
Entry Feedstock
Catalyst
LA
HMF AA
FA GLY
1
2
3
4
5
6
7
8
Glucose
Glucose
Glucose
Glucose
Fructose
Fructose
Fructose
Fructose
Pb–Sn-beta 52.0 6.4
2.8
8.5
4.8 1.2
2.6 6.6
4.1 4.2
Pb-beta
Sn-beta
—
26.5 3.6
22.4 14.4 3.9
0.4 6.7
—
6.7
—
—
Pb–Sn-beta 54.6 4.3
5.9 6.2
Pb-beta
Sn-beta
—
20.6 2.5
40.0 13.5
17.4 43.9
13.3 6.5 5.2
—
—
—
—
1.7 1.7
2.3
3.5 4.3
9
Dihydroxyacetone Pb–Sn-beta 70.0
Dihydroxyacetone Pb-beta
Dihydroxyacetone Sn-beta
—
—
—
—
—
—
—
—
7.5
5.5
—
30.9 5.8
9.2 1.9
21.2 2.6
11.5 3.3
24.8 2.5
—
10
11
12
13
14
15
16
17
18
19
17.7
95.6
21.1
—
—
—
—
—
—
—
Dihydroxyacetone
Pyruvaldehyde
Pyruvaldehyde
Pyruvaldehyde
Pyruvaldehyde
Glyceraldehyde
Glyceraldehyde
Glyceraldehyde
—
Pb–Sn-beta 69.0
Pb-beta
Sn-beta
—
37.0
76.0
26.5
Pb–Sn-beta 56.5 6.6
6.2
11.6 5.0 26.7
9.0
2.2 2.3
Pb-beta
Sn-beta
28.9 3.2
81.3 11.0
—
—
a
Fig. 6 Effect of the lead to tin molar ratio on the yields of main
products with Pb–Sn-beta reaction conditions: 190 ꢁC, 2 h, air, 225 mg
glucose and 200 mg catalyst (0.3 mmol gꢀ1 metal loading). LA: lactic
acid, AA: acetic acid, FA: formic acid, and GLY: glycolic acid.
Reaction conditions: 190 ꢁC, 2 h, air, 10 ml water, 225 mg carbohydrate
(7.5 mmol), 200 mg catalyst (Pb–Sn-beta: 0.3 mmol gꢀ1 metal loading,
Pb/Sn ratio ¼ 4/7; Sn-beta and Pb-beta: 0.3 mmol gꢀ1 metal loading).
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beta, Sn-beta and Pb–Sn-beta zeolites, respectively, for 120 min. whereas it remained nearly constant over Pb-beta zeolite. The
Clearly, compared with the reaction of glucose, the LA yield result implied that the isomerization of glucose to fructose was
from fructose dramatically increased over Sn-beta zeolite, an obstacle for producing lactic acid over the Sn-beta zeolite.
Scheme 1 Proposed reaction mechanism for the conversion of glucose to lactic acid over Pb–Sn-beta.
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However, the Pb–Sn-beta zeolite is advantageous in the isom- A synergistic effect of lead and tin on the conversion of glucose
erization reaction, because the LA yield from fructose was nearly to lactic acid was discovered. The key role of lead was isomeri-
the same as that from glucose. Therefore, incorporating lead zation reaction from glucose to fructose and the retro-aldol
into the Sn-beta zeolite improves ability of isomerization of condensation from fructose to C3 intermediates, while tin
glucose to fructose compared to that over Sn-beta zeolite.
had a superior catalytic performance in the dehydration of
Moreover, in the reaction of dihydroxyacetone, the yield of dihydroxyacetone and isomerization of pyruvaldehyde. In
LA reached 80% and 70% over Sn-beta and Pb–Sn-beta, addition, lead had a positive effect on inhibition mainly by-
respectively, for only 60 min, aer which the yield maintained product HMF.
almost invariant. Compared with the reaction of fructose, the
LA yield was observably increased by Sn-beta; however, the LA
yield was nearly same as that from fructose, approximately 20%
Conflicts of interest
over Pb-beta in 120 min. Therefore, the retro-aldol condensa-
tion of fructose to produce two C3 intermediates has a clear
There are no conicts to declare.
obstacle over Sn-beta, and the incorporation of lead into Sn-beta
decreased the effect of that obstacle. Hence, lead affects the Acknowledgements
retro-aldol condensation from fructose to C3 intermediates
The authors gratefully acknowledge the nancial support
provided by the National Science Fund for Distinguished Young
Scholars (No. 51625804) and the National Natural Science
Foundation of China (No. 21376180).
more than tin. For the reaction of pyruvaldehyde, the LA yield
was 70% and 68% using Sn-beta and Pb–Sn-beta, respectively,
in only 30 min, aer which the yield achieved a plateau. The LA
yield was 37% in 120 min by Pb-beta. Apparently, compared
with reaction of dihydroxyacetone, the LA yield was nearly
invariant with beta zeolite containing tin (Sn-beta and Pb–Sn-
beta), which implied that tin exhibited dramatically efficiency
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