γ-Valerolactone-introduced controlled-isomerization of glucose for lactic acid production over an Sn-Beta catalyst

Article abstract of DOI:10.1039/d1gc00378j

Combined experiments and density functional theory (DFT) calculations provided insights into the role of environment-friendly γ-valerolactone (GVL) as a solvent in the hydrothermal conversion of glucose into lactic acid (LA) over the post-synthesized Sn-Beta catalyst. By introducing 2.0 wt% GVL, a much higher yield of LA (72.0 wt%) was obtained than that in pure water (60.1 wt%) at 200 °C, 4 MPa N2, and 30 min in a batch reactor. The GVL effectively suppressed the isomerization of glucose into fructose in a controlled-transfer mode, resulting in a lower fructose concentration. Thermogravimetry-differential analysis and DFT calculations demonstrated that the competitive adsorption between GVL and glucose happened at the open Sn sites over the Sn-Beta catalyst, which led to a controlled isomerization rate in water. Further increasing the content of GVL to 20.0 wt%, the higher yield of LA (74.0 wt%) was attributed to the more efficient competitive adsorption while also inhibiting carbon deposition.

Full text of DOI:10.1039/d1gc00378j

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γ-Valerolactone-introduced controlled-
isomerization of glucose for lactic acid
production over an Sn-Beta catalyst†
Cite this: Green Chem., 2021, 23,
2634
Received 1st February 2021,
Accepted 15th March 2021
Xinpeng Zhao,‡a,b Zhimin Zhou,‡a,b Hu Luo,a Yanfei Zhang,a,b Wang Liu,a,b Gai Miao,a
Lijun Zhu,a,b Lingzhao Kong,
*
a,b Shenggang Li
*
a,b,c and Yuhan Sun*a,b,c
DOI: 10.1039/d1gc00378j
rsc.li/greenchem
Combined experiments and density functional theory (DFT) calcu- through environment-friendly pathways.5–10 Among the syn-
lations provided insights into the role of environment-friendly thesized catalysts, Sn-Beta zeolite functionalized with frame-
γ-valerolactone (GVL) as a solvent in the hydrothermal conversion work Lewis acidic Sn4+ sites was considered as the state-of-the-
of glucose into lactic acid (LA) over the post-synthesized Sn-Beta art catalyst for the isomerization of glucose to fructose fol-
catalyst. By introducing 2.0 wt% GVL, a much higher yield of LA lowed by the retro-aldolization of fructose to lactic acid.11
(72.0 wt%) was obtained than that in pure water (60.1 wt%) at
Recently, hydrothermal and post-synthesized Sn-Beta zeo-
200 °C, 4 MPa N2, and 30 min in a batch reactor. The GVL effec- lites have been developed to produce LA/methyl lactate (MLA)
tively suppressed the isomerization of glucose into fructose in a from various bio-feedstocks with the highest yield of 68.0% in
controlled-transfer mode, resulting in a lower fructose concen- the water/methanol phase.12,13 In fact, compared to the cata-
tration. Thermogravimetry-differential analysis and DFT calcu- lysts, solvents also have multiple roles including competitive
lations demonstrated that the competitive adsorption between adsorption at reactive sites or (de)stabilization of reactive inter-
GVL and glucose happened at the open Sn sites over the Sn-Beta mediates,11 thus modification of the composition of solvents
catalyst, which led to a controlled isomerization rate in water. is often used to improve the catalytic performance. Padovan
Further increasing the content of GVL to 20.0 wt%, the higher et al. found that the stability of the Sn-Beta catalyst for the con-
yield of LA (74.0 wt%) was attributed to the more efficient com- tinuous upgrading of hexoses into MLA was dramatically
petitive adsorption while also inhibiting carbon deposition.
improved in the water/methanol phase.14 The Sn-Beta catalyst
displayed higher catalytic activity and stability for glucose iso-
merization into fructose in dioxane/water mixtures than in
pure solvents or mixtures thereof, due to the increasing
density of open Sn sites in the presence of water.15
As the sole renewable carbon resource in the world, biomass
can serve as feedstocks to synthesize biofuels and value-added
chemicals through chemical and biological routes.1,2 Among
these products, lactic acid (LA) is an important and versatile
chemical, and is widely used in food, renewable plastics, and
pharmaceutical sectors.3 Traditionally, LA was produced by fer-
mentation of corn starch and syrup, which results in long sep-
aration sequences and a large amount of salt wastes.4 To over-
come these shortcomings, catalysts especially heterogeneous
catalysts with high activity were explored to convert biomass-
derived carbohydrates into lactic acid and/or alkyl lactates
To further improve the reaction rate and product selectivity
for the valorization of biomass in water, γ-valerolactone (GVL),
which is a bio-based green solvent, was introduced into the
reaction of cellulose hydrolysis to glucose,16,17 glucose to
5-hydroxymethylfurfural (5-HMF)18 and levulinic acid.19,20
Song et al. found that the isomerization of glucose into fruc-
tose was suppressed while the rate for fructose dehydration to
5-HMF increased linearly with the addition of GVL to water.17
With the aid of NaX zeolite, the rate of glucose isomerization
greatly declined when water was mixed with 20.0 wt% GVL,
but an increasing rate was observed as the GVL content further
increased, which was attributed to the selective adsorption
and microheterogeneity of the solvent system, respectively.21
Unfortunately, until now there have been no reports on the
conversion of hexoses into LA (from C6 to C3) over an Sn-Beta
catalyst in the GVL/water mixture phase especially regarding
the possible roles of GVL in this reaction.
aCAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210,
P.R. China. E-mail: konglz@sari.ac.cn, lisg@sari.ac.cn, sunyh@sari.ac.cn
bUniversity of Chinese Academy of Sciences, Beijing 100049, P.R. China
cSchool of Physical Science and Technology, ShanghaiTech University, Shanghai
201203, P.R. China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00378j
‡These authors contributed equally.
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In this work, an Sn-Beta catalyst was synthesized through a stability of the Beta zeolite structure during the catalyst syn-
facile top-down strategy, and 2.0 wt% GVL was introduced to thesis process. The micropore volume of the Sn-Beta catalyst
form the GVL/water mixture phase. A higher LA yield of decreased from 0.168 to 0.156 ml g−1 with increasing Sn
72.0 wt% was achieved at 200 °C, 30 min, and 4 MPa N2 when loading, indicating that some extra-framework SnO2 particles
glucose was employed as the feedstock, compared to that of might block the micropores. Similar to that of commercial
60.1 wt% in the pure water phase. Furthermore, probe reac- Beta zeolite, the SEM image (Fig. S2B†) of the 0.75 wt% Sn-
tions, thermogravimetry-differential analysis, and density func- Beta catalyst suggests that the Sn-Beta catalyst consists of
tional theory (DFT) calculations were performed to elucidate spherical particles, suggesting that no collapse of the zeolite
the role of GVL in the valorization of glucose to LA, and the framework happened during the dealumination process. As
reaction mechanism of this reaction in the presence of GVL shown in the UV-vis spectra (Fig. 1B), the peak intensity at
was also proposed based our experimental and computational 210 nm characteristic of tetrahedral coordinated Sn sites
results.
increases with increasing Sn loading, indicating that more Sn
Sn-Beta catalysts with different Sn loadings (0.5, 0.75, 1.5, species were incorporated into the framework. A broader
3.0, and 5.0 wt%), from inductively coupled plasma (ICP) ana- shoulder peak around 280–300 nm was attributed to the
lysis, were first prepared by the post-synthesized method, framework Sn sites in an octahedral coordination with the
details of which can be found in Text S1 of the ESI.† Fig. 1A adsorbed water molecules or the small SnO2 particles.23 As
shows the XRD patterns of Beta, deAl-Beta, and different Sn- shown in the XPS images (Fig. 1C), two dominant signals at
Beta catalysts, which display similar peaks associated with the 487.9 and 496.4 eV are attributed to the 3d5/2 and 3d3/2 orbitals
Beta structure. Notably, no peaks of bulk SnO2 were detected of the tetrahedral coordinated Sn species. In contrast, two
in all these Sn-Beta catalysts, suggesting the high dispersion of signals at 486.0 and 494.5 eV are assigned to the 3d5/2 and
the Sn species over the Beta zeolite support.22 Consistent with 3d3/2 of bulk SnO2. These results also clearly show that Sn
the XRD results, the N2 physisorption data (Table S1 and species were successfully incorporated into the framework
Fig. S1†) show that all the Sn-Beta catalysts have similar even at a high Sn loading of 5.0 wt%. Meanwhile, weak SnO2
Brunauer–Emmett–Teller (BET) surface areas, indicating the signals (631 cm−1) can be detected with Raman spectroscopy
Fig. 1 Characterization of the synthesized samples. (A) XRD patterns of the support and synthesized catalysts. (B–D) UV–vis, Raman, and XPS
spectra of the synthesized catalysts and SnO2.
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for the 5.0 wt% Sn-Beta catalyst (Fig. 1D), suggesting that a
As shown in Fig. 2A, the LA yield presents a volcano shape
small amount of SnO2 species was present as small clusters, as with the increase of the GVL content from 0.0 wt% to
no bulk SnO2 phase was observed using other characterization 98.0 wt% in water. An addition of only 2.0 wt% GVL to water
techniques. As shown in Fig. S3 and Table S1,† Py-IR analysis effectively enhanced the LA yield from 60.1 to 72.0 wt%, and
show that the Lewis acid density of the Sn-Beta catalyst the LA yield further increased to 74.0 wt% when the GVL
increases with increasing Sn content and reaches a plateau of content further increased tenfold. Further addition of GVL
0.13 mmol g−1 above the Sn loading of 1.5 wt%, indicating over 20.0 wt% leads to an obvious decline of the LA yield.
that there was a limit on the amount of Sn species to be Meanwhile, as the main Brønsted acid-catalyzed side products,
inserted into the zeolite framework and the excess tin species the yields of 5-hydroxymethylfurfural (5-HMF) and furfural
was deposited outside of the framework as small SnO2 (FF) increased with the addition of GVL.
clusters.
Typically, the conversion of glucose to LA involves three
As shown in Fig. S4,† the synthesized Sn-Beta catalysts dis- main steps: (1) isomerization of glucose to fructose; (2) retro-
played good catalytic performance with 100.0 wt% conversions aldol condensation of fructose to trioses; (3) dehydration and
of glucose and 60.1 wt% yield of LA at 200 °C for 30 min in further isomerization of trioses to lactic acid. Thus, we
water at the Sn content of 0.75 wt%. When the Sn loading was employed fructose and 1,3-dihydroxyacetone (1,3-DHA) as
above 3.0 wt%, the presence of SnO2 species led to more side probe molecules to further investigate the role of GVL in all
reactions, leading to lower LA yields.24 To further improve the three steps using different solvents (water and 2.0 wt% GVL/
LA yield, GVL was introduced into the reaction system. Blank water mixture). As shown in Fig. 2B, the LA yield was 58.4 wt%
experiments were also performed, and when GVL was used as and 90.6 wt% when fructose and 1,3-DHA served as the raw
the feedstock, no LA was produced and the GVL concentration materials, respectively, in pure water. Unlike glucose, the
was kept constant, demonstrating that GVL was not the source addition of GVL had almost no effect on the yield of LA for
of LA production over the Sn-Beta catalyst.
fructose and 1,3-DHA conversions. So, GVL was supposed to
Fig. 2 (A) Product distributions from glucose in different GVL/water mixtures. (B) Product distributions from fructose and DHA in water and 2.0 wt%
GVL/water mixtures (2.0 wt% GVL(aq)). (C) Product distributions from glucose isomerization reactions in different GVL/water mixtures. (D) Fructose
concentration during glucose conversion in water and 2.0 wt% GVL(aq). Reaction conditions: 0.5 h, 50.0 mg of reactant, 10.0 mL of GVL/water mix-
tures, 4 MPa N2, 100.0 mg of 0.75 wt% Sn-Beta catalyst, 530 rpm. The reaction temperature was 200 °C for (A, B and D) and 100 °C for (C).
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play an important role in the isomerization of glucose to fruc-
tose and the isomerization experiments were evaluated at a
reduced temperature (100 °C) to prevent the subsequent reac-
tions. Fig. 2C shows a dramatic and nonlinear effect of GVL
and the isomerization rate decreased by 49.3% with the
addition of only 2.0 wt% of GVL in comparison to that when
only water was present. When the content of GVL further
increased to 20.0 wt%, the glucose isomerization rate
decreased by 80.4%. Surprisingly, the isomerization rate was
recovered with a further increase of the GVL content. At
70.0 wt% of GVL, the isomerization rate was even higher than
that in pure water. Comparing the corresponding isomeriza-
tion rate and LA yield in different GVL/water mixtures (Fig. 2A
and C), we found that more LA could be obtained when the
isomerization reaction was carried out at a low rate. It has pre-
viously been reported that the formation of polymers (such as
humins) during the hydrothermal synthesis greatly decreased
the utilization efficiency of carbohydrates,25 and self-polymer-
ization of HMF or co-polymerization between HMF and fruc-
tose was supposed to be the sources of polymer (such as
humins) formation.26 As shown in Fig. S5,† co-polymerization
of HMF and fructose was reduced with the decrease of fructose
concentration. So, a modest amount of GVL should be intro-
duced to restrain the isomerization rate of glucose to fructose
in a controlled-reaction mode, which will keep the fructose
concentration at a relatively low level to favour the reaction at
the active sites over the Sn-Beta catalyst and avoid the side
reaction between fructose and HMF. The continuous sampling
experiment (Fig. 2D) also revealed that the fructose concen-
tration in the 2.0 wt% GVL/water mixture was lower than that
in pure water during the reaction. The above reasoning was
further confirmed when a LA yield of 80.0 wt% or 77.3 wt%
Fig. 3 TG-DTG profiles for different Sn-Beta catalysts pre-adsorbed
with 16.6 wt% GVL.
was obtained with 1.0 g L−1 fructose or glucose as the feed- its peak intensity increased with increasing Sn content and
stock (Fig. S6†). The stability of the Sn-Beta catalyst was also reached a maximum above 3.0 wt%, which also indicates the
investigated in water and 20.0 wt% GVL/water mixture. As saturation of the amount of inserted Sn. The DTG peak at
shown in Fig. S7,† the LA yield of 58.1 wt% was maintained 277.7 °C was only present for the Sn-Beta catalysts with Sn con-
after five runs with catalyst regeneration in water. tents above 1.5 wt%, indicating that these peaks could be
Furthermore, the catalyst activity was retained in the 20.0 wt% attributed to the GVL adsorbed over SnO2 clusters. TPD charac-
GVL/water mixture and a LA yield of 71.0 wt% was maintained terization of deAl-Beta was also performed as shown in
after five cycles with catalyst regeneration. Furthermore, XRD Fig. S9.† Apart from the desorption signals of the physically
images (Fig. S8†) for the used and regenerated catalysts show adsorbed water and GVL, the peak centred at 361.1 °C suggests
that the structure of the Sn-Beta catalyst remained during the that the chemical adsorption between deAl-Beta and GVL was
conversion of glucose into LA.
stronger than that in the Sn-Beta catalyst, probably due to a
To further elucidate the role of GVL in the isomerization stronger interaction between silanol nests and GVL.27 The
process, thermogravimetry (TG)-differential analysis was used peak in DTG centred at 600 °C in deAl-Beta was attributed to
to obtain temperature-programmed desorption (TPD) profiles the loss of water from the condensation of silanol nests.28
of the Sn-Beta catalysts pre-adsorbed with GVL molecules (see Thus, we consider that GVL has a strong tendency to coordi-
the ESI† for details of the methods). As shown in Fig. 3B, the nate with framework Sn sites of the Sn-Beta catalyst. UV-vis
desorption peaks below 100 °C in all samples were attributed spectroscopy of 5.0 wt% Sn-Beta and its GVL-adsorbed mixture
to water adsorbed in the zeolite. The peaks in DTG centred at (Fig. S10†) further shows that GVL can coordinate with the
207.7 °C for all samples were due to the evaporation of the es framework Sn species to form an octahedral coordinated
GVL. The fact that the desorption peaks of GVL are higher framework Sn species.27,29 Recent studies suggest that the par-
than its boiling point for all samples accounts for the quanti- tially hydrolyzed open lattice sites are the active centers for
tative chemisorption of GVL on the Sn-Beta catalyst. The DTG sugar isomerization over the Sn-Beta catalysts, and competitive
signal at 319.2 °C was assigned to GVL adsorbed at the frame- adsorption between the solvent and sugars affects the catalytic
work Sn sites due to its presence in all Sn-Beta catalysts, and performance.30–32 Thus, the adsorption energies of the
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different species present in the reactions were obtained from glucose leads to a low coverage of glucose at the Sn active sites,
DFT calculations. thus reducing the isomerization reaction rate and leading to a
The adsorption configurations (Fig. 4 and Fig. S12†) were lower fructose concentration in water. With the increase of the
constructed following the results of a previous theoretical GVL content to 20.0 wt%, GVL molecules surround fructose
study.31 As shown in Fig. 4, the open Sn site tends to bind two and side products (5-HMF and FF) as the solvent in the zeolite
water molecules, or two GVL molecules, or one glucose mole- pore, thus carbon deposition was avoided resulting in an
cule. The strongest adsorption was predicted for GVL (−2.15 increased yield of the total liquid products (Fig. S13†).
eV) among all adsorbed species, which would inhibit glucose
adsorption at the open Sn sites when glucose, water, and GVL
competed for adsorption at the open Sn site with a low GVL
content below 20.0 wt% in the reaction mixture, thus leading
Conclusions
In summary, we have demonstrated the dramatic and nonlinear
effect of GVL as a green solvent on glucose conversion into LA
over a post-synthesized Sn-Beta catalyst in water. An LA yield of
to a reduced sugar isomerization reaction rate. In contrast, a
microscale-heterogeneous solution was formed when the GVL
content increased above 20.0 wt%, and the solvent microheter-
74.0 wt% was obtained at 200 °C, 4 MPa N2, and 30 min. GVL
ogeneity would cause water associated with glucose to be con-
effectively inhibited the isomerization of glucose into fructose
centrated in the zeolite pore, thus leading to the recovery of
the glucose isomerization rate21 and the increasing rates of the
through a controlled-transfer mechanism, which leads to an
efficient contact between fructose at a relatively low concen-
side reactions as shown in Fig. 2A and C. According to the
tration with the Sn-Beta catalyst. DFT calculations provided
above theoretical analysis, we illustrate the role of GVL in the
further evidence that GVL competes with glucose to access the
conversion of glucose to lactic acid under the different ratios
of GVL/water in Scheme 1. At a relatively low content of GVL,
Sn active sites especially below 20.0 wt% of the GVL/water
mixture. The present results indicated that GVL is a suitable
competitive adsorption served as the dominant mechanism in
additive favourable for the synthesis of LA through competitive
the whole process. Hydrophilic Sn-Beta pores tend to bind
adsorption along with carbon deposition prevention.
GVL molecules, and competitive adsorption between GVL and
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
Financial support from the National Natural Science
Foundation of China (21978316) and the Ministry of Science
and Technology of China (2018YFB0604700) is acknowledged.
Fig. 4 Structures and adsorption energies (ΔE) of two molecules of
water and GVL, and one molecule of glucose at the open Sn site in the
Sn-Beta catalyst. For clarity, only the relevant part of the periodic zeolite
model is shown (bond lengths in Å).
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