Synergetic effect of Lewis acid and base in modified Sn-β on the direct conversion of levoglucosan to lactic acid

Article abstract of DOI:10.1039/d0cy00089b

This work presents a novel method to selectively convert levoglucosan to lactic acid (LA) via initially adding LA as the Br?nsted acid together with modified Sn-β as the Lewis acid. Sn-β catalysts exchanged with various cations significantly enhance the yield of LA, following the order Sn-β < Sn-β-Na < Sn-β-K < Sn-β-Mg, Sn-β-Ca. The role of exchanged alkali ions stems from removing the remaining Br?nsted acid sites in the zeolite and tuning the framework oxygen with more Lewis base sites. The exchange with alkaline earth cations such as Ca²⁺ and Mg²⁺ appears to be more effective in promoting the retro-aldol condensation due to the synergetic effect of their stronger Lewis acid that can stabilize the oxygen atom of deprotonated alkoxides during the reaction. The LA yield from levoglucosan can reach up to 66% on Sn-β-Ca under the optimized conditions. We also demonstrated that such a synergetic effect of alkaline-earth ion exchanged Sn-β shows great universality toward enhanced retro-aldol condensation, which can significantly promote the yield of methyl lactate over glucose conversion in methanol solvent.

Full text of DOI:10.1039/d0cy00089b

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ISSN 2044-4761
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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D0CY00089B
ARTICLE
Synergetic effect of Lewis acid and base in modified Sn-β on direct
conversion of levoglucosan to lactic acid
Received 00th January 20xx,
Accepted 00th January 20xx
Wenda Hu, a,c Zixin Chi, a Yan Wan, a Shuai Wang, a Jingdong Lin, a Shaolong Wan *a, b and Yong
Wang *a, c
DOI: 10.1039/x0xx00000x
This work presents
a
novel method to selectively convert chemical widely used in the food, pharmaceutical, cosmetic
levoglucosan to Lactic acid (LA), via initially adding LA as Brønsted and chemical industries .9-11 Currently, LA is primarily produced
acid together with modified Sn-β as Lewis acid. Sn-β catalysts by the fermentation of glucose, which is limited by the
exchanged with various cations significantly enhance the yield of drawbacks such as low efficiency of biological process and a
LA, following the order of Sn-β< Sn-β-Na< Sn-β-K< Sn-β-Mg, Sn-β- plethora of waste accompanied.12 Hence, substantial studies
Ca. The role of exchanged alkali ions stems from removing the have been dedicated to the catalytic thermochemical
remaining Brønsted acid sites in the zeolite and tuning the processes for LA production from different carbohydrates such
framework oxygen as more Lewis base sites. The exchange with as triose sugars,13-15 monosaccharide and polysaccharide.12, 16-
17
alkaline earth cations such as Ca2+ and Mg2+ appears to be more
Accordingly a
variety of efficient homogeneous13-16 or
effective in promoting the retro-aldol condensation, due to the heterogeneous catalysts20-22 have been developed, among
synergetic effect of their stronger Lewis acid that can stabilize the which much interest has been paid to Sn-β catalysts due to
oxygen atom of deprotonated alkoxide during the reaction. The LA their strong and water-tolerant solid Lewis acid properties.18-24
yield from Levoglucosan can reach up to 66% on Sn-β-Ca under the Moreover, modified Sn-β zeolites, such as Zn-Sn-β, Sn-β
optimized conditions. We also demonstrated that such
a
exchanged with alkali metal, and hierarchical Sn-β, have been
synergetic effect of alkaline-earth ion exchanged Sn-β shows great reported to increase the selectivity of LA or its derivates.12, 25-26
universality toward enhanced retrol-aldol condensation, which The advances achieved over post-synthetic methods also
can significantly promote the yield of methyl lactate over glucose enable more efficient preparation of Sn-β catalysts with higher
conversion in methanol solvent.
framework-Sn content compared to the conventional
22, 27-31
hydrothermal synthesis routes.13,
However, most of
1 Introduction
these heterogeneous practices are limited to the conversion of
a major product from fast pyrolysis of triose or C6 monomer sugars that have to be produced through
Levoglucosan is
abundant lignocellulose biomass, which accounts for 23 wt% costly chemical processes,9-10 and are not efficient in the direct
yield with sugar cane bagasse as the feed,1 or even reaches up conversion of cellulose or raw lignocellulose. Therefore, it
to 60 wt% upon using isolated cellulose.2-3 It provides an would be highly desired to selectively produce LA from simple
important platform molecule for the synthesis of a variety of oxygenates like acetol32 or levoglucosan, which can be readily
value-added chemicals.4-8 Santhanaraj et al. reported a two- produced at large scale from staged condensation or pyrolysis
step method for the production of gluconic acid from a high of abundant raw biomass.
purity solution of levoglucosan.5 Herein, we target its selective Levoglucosan is a type of dehydrated C6 sugar monomer,
conversion to another important chemical, namely lactic acid which can be readily hydrolyzed to form glucose using
(LA). LA is
a valuable monomer for the production of Brønsted acid catalysts. Once glucose is produced, Lewis acid
biodegradable plastics, as well as an important building-block catalysts such as Sn-β can be applied for its subsequent
conversion toward LA.33 It appears quite straight forward that
a combination of Brønsted and Lewis acid sites can potentially
enable such a tandem conversion. However, an initial attempt
a. College of Chemistry and Chemical Engineering, Xiamen 361005, China.
b. National Engineering Laboratory for Green Chemical Productions of Alcohols -
Ethers-Esters, Xiamen 361005, China.
Voiland School of Chemical Engineering and Bioengineering Washington State
University, Pullman, Washington 99164, United States.
Electronic Supplementa ry Inf ormation (ESI) available: [details of any
supplementary information available should be in cluded he re]. See
DOI: 10.1039/x0xx00000x
for such a tandem conversion showed that the presence of
Brønsted and Lewis acids can catalyze very complex reactions,
leading to low yield of the desired product.34 To circumvent
such a dilemma, we report a novel method for the conversion
of levoglucosan, where external LA is added as the Brønsted
acid and the modified Sn-β was employed as the Lewis
c.
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counterpart. Since LA itself is the targeted product, there will different response intensity (Fig S2b), suggesting that
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be no need for post-reaction separation caused by using the exchanged cations do not insert into the DfrOaIm: 1e0w.1o0r3k9/.D0CY00089B
homogeneous acid. The stratagem here also simplifies the The state of Sn species in Sn-β samples was also studied by
catalyst design, with the focus of only enhancing the Lewis XPS, as shown in Fig S3. The spectrum displays signals of the Sn
acidity of Sn-β catalysts as opposed to simultaneously 3d3/2 and 3d5/2 binding energies at 495.8 and 487.4 eV, which
introducing both Brønsted acid and Lewis acid sites. The post- are typically ascribed to tetrahedrally coordinated framework
36
synthesis Sn-β prepared in this work was further modified with Sn Species.27,
ion-exchange of various alkali and alkaline-earth cations to
fine-tune the Lewis acid-base properties inside the confined
space, achieving the synergetic effect in enhancing LA yield.
2 Results and discussion
2.1 Catalyst characterization
The topology of beta zeolite remained intact after all the
treatments, including acid treatment, incorporation of metal
and exchange with various cations (Fig.S1). Moreover, no
diffraction peaks corresponding to SnO2 (JCPDS No. 41-1445)
exist in Sn-β, indicating the little formation of extra-framework
SnO2 (at least below the detection limit of XRD).34 The
reflection peaks of Sn-β remain constant after exchanging with
Figure 1. CO2-TPD of Sn-β and exchanged Sn-β
various cations, suggesting no destruction of the zeolite
structure. The surface areas of all the zeolites didn’t change
significantly after acid treatment and exchanging treatment, as
Catalyst’s basicity was characterized by CO2-TPD. As shown in
Fig. 1, the basicity of Sn-β was evidently enhanced after
shown in Table 1. The contents of metal exchanged onto the
exchanging with different cations. The strength of catalyst
zeolite are also measured and listed in the same table.
basicity follows the order of Sn-β< Sn-β-Mg< Sn-β-Ca< Sn-β-
Na< Sn-β-K, which is similar to the results reported
Table 1 physiochemical properties of different zeolites
previously.23, 37
Surface
Content of metal[b]
Molar ratio
Entry
Catalyst
area[a]
[m2g-1]
460
2.2 External lactic acid initially added as the Brønsted acid for
hydrolysis
1
2
3
4
5
6
7
H-β
deAl-β
Sn-β
Sn-β-Na
Sn-β-K
Sn-β-Mg
Sn-β-Ca
-
498
467
437
489
444
403
-
Si/Sn≈18[c]
Sn/Na≈11.4
Sn/K≈11.1
Sn/Mg≈14.9
Sn/Ca≈20 (10[d]
)
aSpecific surface areas obtained by BET method.
bDetermined by ICP.
CDecided by EDX.
dExchanged for three times.
Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy
was used to understand if Sn was incorporated in framework
positions. Relatively low absorption in deAl-β suggests that
little electronic transition occurred (Fig. S2a). The presence of
tetrahedrally coordinated Sn in the framework Sn-β zeolites is
confirmed by an absorption peak near 200 nm due to the
Figure 2. Time on stream of conversion of levoglucosan with Sn-β.
Reaction conditions: cat. 200 mg; lactic acid 0.2 mmol; levoglucosan
0.05 g; H2O 20 mL; N2 2 MPa; temperature 190 oC.
ligand-to-metal charge transfer from O2- to Sn4+ 13, 27, 35
SnO2 is
.
Levoglucosan conversion was initially attempted with the use
of Sn-β only, where lactic acid was found to be a major
product with a yield about 30% (Fig.S4). However, it took more
than 6 hours to reach complete conversion, even when the
temperature was raised to 190 oC. In contrast, the introduction
of external lactic acid (0.2 mmol) significantly speeds up
reaction, resulting in complete conversion in less than 2 hours
present in SnO2/deAl-β as evidenced by a broad absorption
from 250 nm-300 nm.27 the fact that there is no SnO2 phase in
Sn-β zeolites confirms that most Sn has been successfully
incorporated into the framework of Sn-β zeolites, which is
consistent with our earlier work.34 Cation-exchanged Sn-β
zeolites have the same absorption peak except for slightly
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with even improved selectivity of LA. This provides us a piece Sn-β-Na indeed demonstrated enhanced LAViewsAertlieclcetOivnilitnye
of direct evidence that lactic acid i.e. the target product self compared to that of parent Sn-β iDnOIt:h1e0.10c3o9n/Dv0eCrsYi0o0n089oBf
can effectively function as the Brønsted acid to facilitate the levoglucosan, as shown in Fig. 3. Accordingly, the side products
hydrolysis of levoglucosan, which appears apparently as the such as formic acid were notably inhibited.
rate-relevant step in the case of only using Sn-β. The temporal
evolution of products from the combination of Sn-β and extra
lactic acid is shown in Fig. 2. LA exhibited a steady increase in
yield with time-on-stream, reaching 34% after
reaction. Accordingly, the yield of glucose and fructose
exhibited typical trend of intermediate products, both
4 hours’
a
peaked at around 15% after 25 min and then decreased
gradually until complete disappearance. This result is
consistent with the postulated reaction pathway depicted in
scheme 1a, which proceeds via the isomerization and
subsequent decomposition of these two hexoses respectively.
However, severe side reactions took place, leading to such a
low yield of LA. Among them, a small amount of HMF
derivatives (5-(hydroxymethyl furfural and levulinic acid) were
produced, apparently due to dehydration of fructose and
hydrolysis of HMF, respectively, catalyzed by Brønsted acid.
Some decomposition products (formic acid and acetol) were
also found, which is consistent with the previous reports.12, 17
Moreover, a significant amount of oligomers were detected,
which exhibit the identical peaks with the products generated
when only using HMF as a reactant by liquid chromatography-
mass spectrometry (LC–MS). This suggests that these heavy
products are mainly derived from HMF, a compound known
very vulnerable to rapid condensation and polymerization. 38
The effect of reaction temperature on the conversion of
levoglucosan was also studied. At a low reaction temperature
of 150 oC, only a small amount of LA was obtained, leaving
glucose and fructose accounted for the most dominant
products after 4 hours time-on-stream (Fig.S5). This suggests
retro-aldol condensation of hexose to triose sugars is the rate-
limiting step,22 which would be facilitated at higher
temperatures. Indeed, the yield of LA was significantly
Figure 3. Conversion of levoglucosan with different ratio of Lewis acid
and base. Reaction conditions: levoglucosan 0.05 g; H2O 20 mL; N2
MPa; temperature 190 oC; time 2 h.
2
The ratio of Lewis acid and extra-added Brønsted acid was
further studied and results are shown in Fig. 3. By regulating
the Lewis acid and external LA added, we show that increasing
the amount of Sn-β-Na from 200 mg to 300 mg can notably
improve the selectivity of LA, while the side products such as
HMF and levulinic acid are significantly suppressed. Further
increase of Sn-β-Na to 500 mg with 0.2mmol extra LA does not
show appreciable enhancement of LA yield but reduces HMF
and levulinic acid to a negligible extent. It appears that the
combination of 0.3g Sn-β-Na and 0.2 mmol external LA can
achieve the optimal performances. However,
a further
increase in external LA (from 0.2 mmol to 0.4mmol) would
only cause the drop of LA yield and more severed side
reactions toward heavy products. Hence, the subsequent
comparisons among various cation-exchanged Sn-β catalysts
were all conducted with the addition of external LA fixed at
0.2mmol.
o
enhanced when the reaction temperature increased to 170 C.
Further increase of temperature to 190 oC resulted in similar
product distribution, but requiring half of reaction time for the
complete conversion (about 2h). Hence, the subsequent tests
over levoglucosan were conducted at 190 oC, where the
combination of external lactic acid and modified Sn-β was
studied to evaluate their synergetic effect.
2.3 Sn-β exchanged with various cations
As discussed above, LA was successfully produced from
levoglucosan, but with a rather low yield, no matter whether
Sn-β was used alone or was combined with external LA. Our
previous work suggested that the acid properties inside the
confined zeolite pores play important roles over the reactions
of methacrolein and ethanol on Sn-β, where the removal of
weak Brønsted acid sites derived from the remaining silanol
groups could significantly enhance the target products that are
catalyzed by the strong Lewis acid sites.34 Alkali cations such as
Na+ have also been commonly used to exchange the protons of
silanol in Sn-β, thus eliminating the weak Brønsted acid and
enhancing the LA yield in the conversion of glucose.25 Similarly,
Figure 4. Conversion of levoglucosan by using different modified Sn-β.
Reaction conditions: cat. 200 mg; lactic acid 0.2 mmol; levoglucosan
0.05 g; H2O 20 mL; N2 2 MPa; temperature 190 oC; time 2 h.
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The more striking effect was even achieved when the same 2.4 Synergistic effect between Lewis acid and base of Sn-β
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DOI: 10.1039/D0CY00089B
approach aforementioned was extended to other alkali cations toward retro-aldol condensation
such as K+ and alkaline earth cations such as Ca 2+ and Mg2+, Overall, the results above demonstrated that the combination
which are capable of fine-tuning the acid-base properties of external LA and Sn-β exchanged with alkali/ alkaline earth
inside the confined zeolite environment. It can be seen in Fig. 4 cations led to the efficient conversion of levoglucosan into LA,
that the yield of LA for Sn-β-K is considerably higher than that which follows the pathway depicted i n Scheme 1a. It’s quite
for Sn-β-Na, while those of Sn-β-Ca and Sn-β-Mg are even straightforward that the introduction of an appropriate
better. The results achieved from the exchange of alkaline amount of external LA should work as added homogenous
earth cations appear quite striking, considering their less molar Brønsted acid to facilitate the desired hydrolysis of
content (Sn/Mg≈14.9 and Sn/Ca≈20) compared to that of Na levoglucosan, while excess LA would enhance the side
or K amount (Sn/Na≈11.4 and Sn/K≈11.1) exchanged in the reactions leading to diminished LA yield. On the other hand,
modified zeolite as shown in Table 1. Accordingly, the Sn-β exchanged with either alkali or alkaline earth cations will
employment
of
Sn-β-Ca-3 with comparable
cation eliminate the remaining weak Brønsted acid, thus appearing
concentration as that in Sn-β-Na or Sn-β-K can further enhance beneficial to inhibit the side reactions inside the confined pore
the yield of LA, which eventually reaching up to 66% under environment. Apparently, acid-base properties inside the
optimized condition (i.e. 0.3g Sn-β-Ca-3 and 0.2 mmol LA).
confined pore area of zeolite and Brønsted acid in the bulk
aqueous solution played key roles in achieving maximum
performances. The hydrolysis step of levoglucosan probably
takes place dominantly in the bulk aqueous solution, which is
facilitated by the homogeneous Brønsted acid introduced by
the external LA added. Isomerization of glucose and
subsequent retro-aldol condensation proceed mainly inside
the zeolite pores on the strong Lewis acid site, where the
neutralization of neighboring weak Brønsted acid by cations
exchanged helps inhibit the side reactions as shown in Fig. 3.
The somewhat hydrophobic property of Sn-β would restrict
the diffusion of aqueous bulk LA into the pore, thus limiting
homogenous Brønsted acid inside the pores.
The more intriguing part lies in the distinguished effect
between exchanged alkali and alkaline metal cations, where
the latter exhibit more significantly enhanced selectivity
toward LA. To shed light on the insight of the reaction
mechanism, we employed the glucose and fructose
Figure 5. Conversion of levoglucosan by adding different amount of intermediates as the reaction substrates, respectively, where
Ca2+
Reaction conditions: cat. 200 mg; lactic acid 0.2 mmol; only the various Sn-β catalysts were used as catalysts and the
levoglucosan 0.05 g; H2O 20 mL; N2 2 MPa; temperature 190 oC; time 2 catalytic performances were intentionally carried out at a low
.
h.
temperature of 150 oC to better reveal the distinct effects on
the retro-aldol reaction. As shown in Fig. S6a, both the Na, K,
Great care was taken to ensure the origin of the catalytic and Ca-exchanged Sn-β resulted in somewhat loss of activity
effect is indeed from the heterogeneous active sites. No cation over the conversion of glucose, but with a different trend of
was detected in the post-reaction solution, indicating no product distributions. Sn-β-Na has higher selectivity of
catalyst leaching during the reaction. We then conducted a fructose, while barely showing promoting effect toward that of
comparison test by using the combination of unmodified Sn-β LA. Instead, Sn-β-K exhibits a comparable yield of fructose, but
and homogenous Ca2+ (in the form of Ca(NO3)2). As shown in a notable increase of LA yield, which is further enhanced upon
Fig. 5, almost 6 times higher amount of homogeneous Ca2+ the employment of Sn-β-Ca. A similar conclusion can be drawn
added can reach a comparable yield of LA. Much higher with the conversion of fructose (Fig. S6b). The selectivity of LA
homogeneous Ca2+ added results in a higher yield of LA (69%) upon Sn-β-Ca is significantly higher than that of Sn-β-K and
attributed to a higher amount of Ca2+ exchanged onto the Sn- unmodified Sn-β, while even some decline was evident over
β. This confirms that the catalytic effects are indeed from the that of Sn-β-Na. The kinetic-relevant step of LA production
exchanged cations in zeolites, rather than the homogenous from either glucose or fructose is related with the retro-aldol
ones leached into the solution during the reaction.
condensation, as mentioned earlier. All these clearly suggest
that Sn-β exchanged with Ca and K can catalyze this key
reaction, while the counterpart modified with Na seems to lack
of such catalytic effect at low temperature.
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It should be mentioned that the synergistic effect of alkaline-
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earth ion exchanged Sn-β shows greaDtOuI:n10iv.1e0r3s9a/lDit0yCYt0o0w0a89rBd
enhanced retro-aldol condensation. Such a mechanism is not
only limited to the conversion of levoglucosan or glucose to
lactic acid in aqueous solution, but also being effective in
glucose conversion to methyl lactate (MLA) in methanol
environment. As shown in Fig. 6a, the alkaline earth metal
exchanged Sn-β presents a much better yield of MLA than the
bare one. The yield of MLA increases from 21 to 33% and 35%
when Sn-β exchanged with Ca and Mg respectively, suggesting
the synergetic effect of Lewis acid-base also plays an
important role in boosting the retro-aldol condensation in an
alcohol solvent.
The stability of catalysts in varied solvents has also been
carefully evaluated. As shown in Fig. S7, the activity decreased
drastically over 3 consecutive runs for the conversion of
levoglucosan to lactic acid at 190 oC. The decreased
crystallinity of catalysts after reacting in hydrothermal
conditions39-40 and the leaching of Ca2+ in the aqueous acidic
solution should be the two relevant factors toward such a
drastic loss of reactivity. Indeed, the content of Ca2+ in the
catalysts was substantially declined from 0.3wt% initially to
0.13wt% just after the 2nd cycle. Even when the reaction
temperature was lowered from 190 oC to 150 oC, the
deactivation was still quite significant, where no apparent
destruction of crystal structure was observed from the XRD
result of the spent catalyst (Fig. S8). Thus, the deactivation at
relatively low reaction temperatures should be mainly caused
by the leaching of metal cation, which was reversely
exchanged with H+ derived from lactic acid initially added or
gradually produced during the reaction. This suggests that the
modified Sn-β catalysts need to be frequently refreshed via ion
exchange to make up the leached cation after certain batches
of reaction, if applied into practical application for the
conversion of levoglucosan or glucose to lactic acid in aqueous
solution. Alternatively, one possible solution would be to
recycle the aqueous solution with leached metal ions, which
can be readily separated after the collection of spent catalyst
and the recovery of organic acid products. The application of
such recycled solution with concentrated alkaline-earth metal
ions would be beneficial to the yield of LA (as shown in Figure
5), and meanwhile limit the extent of ion exchange between H+
in the bulk solution and Ca 2+ in the modified Sn-β catalysts,
thus alleviating the deactivation caused by metal ion leaching.
However, this stability dilemma confronted in aqueous
solution appears not a big concern for the production of
methyl lactate from glucose in an organic solvent like
methanol. As demonstrated in Fig. 6b, the yield of methyl
lactate remained comparable during 5 consecutive runs with
Sn-β-Mg, suggesting the good stability of the catalyst in
methanol solvent. Accordingly, there was no significant
leaching of Mg2+ in the recycled catalysts, apparently thanks to
the avoidance of organic acid products and the use of an
organic solvent that leaves H+ barely available in the bulk
solution to exchange with alkaline earth cation of the catalyst.
Scheme 1. a) Reaction pathway in the conversion of
levoglucosan to LA. b) Reaction mechanism over alkaline-earth
ion exchanged Sn-β
Considering these results, the possible mechanism of
enhanced retro-aldol condensation over alkaline-earth ion
exchanged Sn-β is then tentatively proposed. As depicted in
scheme 1b, the ketonic oxygen of fructose adsorbs onto
tetrahedrally coordinated Sn sites first, and the C-C bond is
subsequently cleaved to form C3 species, such as
dihydroxyacetone and glyceraldehyde that dehydrate and
undergo 1,2-hydride shift to form LA. It is probably the
synergistic effect of the frustrated Lewis acid and base pair,
that plays an important role to promote the reaction.
Specifically, the Lewis base site, namely framework O atom of
Si-O-M (Ca2+ or Mg2+), can help abstract the proton of
a
hydroxyl group attached to C3 of fructose, while Ca2+ or Mg2+
can stabilize the deprotonated alkoxide, thanks to the
relatively strong Lewis acidity.37 Such stabilization from the
alkaline-earth cation could then facilitate the cleavage of C-C
bond and subsequently form LA through two trioses
intermediates. It should be noted that the alkali cations
exchanged zeolites actually bear stronger Lewis base sites, as
revealed in Fig. 1. Similar results have been reported that the
replacement of Na+ with heavier alkali metal ions like K+ , Rb+
and Cs+ would further increase the base strength of the
adjacent framework oxygen atoms, while the exchange with
alkaline-earth ion like Ba2+ in zeolites results in lower base
strength of such oxygen atoms, but leading to stronger Lewis
acid site due to the higher positive field of the divalent
cations.37 Herein, the presence and the lack of promotional
effect toward LA over Sn-β-K and Sn-β-Na respectively suggest
that only the stronger basic sites promote the retro-aldol
condensation, while the superior effect from Sn-β-Ca is more
likely from the stronger Lewis acidity derived from the
alkaline-earth cation, which synergistically cooperate with the
frustrated base O atom to stabilize the transition intermediate
and accelerate the retro-aldol condensation.
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applied to the conversion of glucose in methanol solvent,
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where Sn-β exchanged with Ca and Mg DsiOgIn:i1f0ic.1a0n3t9l/yD0eCnYh0a0n0c8e9dB
the yield of methyl lactate and retained good stability during
the recycle tests. Drastic deactivation occurred in the
conversion of levoglucosan, mainly due to the severe leaching
of alkaline earth cation in the aqueous acidic solution.
4 Experimental
4.1 Chemicals and materials
cellulose, glucose, fructose, mannose and 5-hydroxymethyl
furfural were obtained from Aladdin Reagent Co. Ltd.
(Shanghai, China). Tin (II) acetate (99%), levoglucocan (99%)
were purchased from J&K Chemical Technology. Lactic acid
(85.0%-90.0%), acetol (95.0%) and inulin were obtained from
Alfa Aesar. Sodium nitrate (99.0%), potassium nitrate (99.0%),
calcium nitrate (99.0%), magnesium nitrate (99.0%) and
commercial H-β zeolite (Si/Al=12.5), HNO3 (65%-68%) and
formic acid (98%) were supplied by Sinopharm Chemical
Reagent Co. Ltd. (Shanghai, China). N2 (99.999%) was obtained
from Linde Industrial Gases. Levulinic acid (99%) was supplied
by Macklin Reagent Co. Ltd. (Shanghai, China). HF was supplied
by Xilong Scientific Co. Ltd. (Shantou, China). Ballmilled
cellulose was prepared via ball-milling the microcrystalline
cellulose at 60 rpm for 48 h by using a QM-3S P04 planetary
ball miller.
4.2 Catalyst synthesis
The preparation of zeolites Sn-β follows the similar methods
Figure. 6 a) Synergetic effect of Lewis acid base on glucose
conversion to methyl lactate. b) Recycling test of glucose
conversion to methyl lactate over Sn-β-Mg. Reaction
previously reported by Hermans and Hammond et al.30-31
Specifically, H-β (Si/Al=12.5) was dealuminated by treatment in
HNO3 solution (65%-68% HNO3, 373K, 20 h, 20 mLg-1 zeolite). The
acid-treated powder was then filtered, washed thoroughly with
deionized water, and dried at 383 K overnight. The dealuminated
zeolite (denoted as deAl-β) was subsequently grafted with tin with
the solid-state ion exchange (SSIE) method, i.e. by grinding with the
appropriate amount of tin (II) acetate in a pestle and mortar. The
obtained mixing powder was finally calcined in a tubular reactor to
obtain the Sn-β, which was heated up to 823 K (10K min-1 ramp
rate) firstin a flow of N2 (6 h) and subsequentlyin air (3h) for a total
of 9 h. A series of ion-exchanged Sn-β were prepared post-
conditions: cat. 400 mg; glucose 0.055 g; methanol 20 mL; N2
MPa; temperature 160 C, time 4 h.
2
o
3 Conclusions
A combination of post-synthesis modified Sn-β as Lewis acid
and externally added LA as Brønsted acid has been shown to
be effective in the selective conversion of levoglucosan to LA.
The initial introduction of the product LA can efficiently
catalyze the first hydration step to accelerate the overall synthetically, where the calcined Sn-β was soaked into a 1.0 M
process without sacrificing the net LA yield. Sn-β exchanged M(NO3)n solution (50 mLg-1 zeolite, M= Na+, K+, n=1; M= Ca2+, Mg2+
with alkali ions can neutralize the remaining Brønsted acid
sites and tune the framework oxygen as stronger Lewis basic
n=2) and the mixture was stirred at 353 K for 12 h. The solid sample
was recovered by filtration and washed with dis tilled water. After
being dried, the sample was calcined at 773 K for 5 h, named Sn-β-
M. Sn-β exchanged with MNO3 for one time is denoted as Sn-β-M,
three times as Sn-β-M-3. The deAl-βsupported tin oxides (denoted
as SnO2/deAl-β) were also prepared through solid state mixing of
sites.
The
reactivity-structure-relationship
analysis
demonstrated that Sn-β-Na can benefit the LA yield probably
via suppressing the side reactions, while Sn-β-K can effectively
facilitate the retro-aldol condensation. The exchange with
alkaline earth cations such as Ca2+ and Mg2+ can further
enhance the selectivity to LA, which may result from the
synergetic effect of their stronger Lewis acid that can stabilize
the oxygen atom of deprotonated alkoxide during the
conversion of fructose intermediate. The optimum yield of LA
can reach up to 66% in the case of Sn-β-Ca-3. The same
deAl-β with SnO2, followed bycalcination at 823 K for 12 h.
4.3 Catalyst characterization
The surface areas of the samples were measured by nitrogen
physisorption on a Micromeritics ASAP 2020 M instrument. Before
the adsorption, samples were degassed at 573 K for 3h. The total
synergistic effect on retro-aldol condensation can be also surface area was calculated via the Brunauer-Emmett-Teller (BET)
6 | J. Name., 2012, 00, 1-3
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equation. Scanning electron microscope (SEM) was conducted by
Zeiss sigma microscope, with Energy Dispersive X-ray Spectroscopy
(EDS) measurements analyzing the chemical compositions. X-ray
diffraction (XRD) patterns of samples were recorded on Rigaku
UItima IV X-ray diffractometer with CuKα radiation source (40kV
and 30mA) from 5o to 60 o and a scan speed of 2θ=10.0o /min.
Inductively coupled plasma-optical emission spectrometry (ICP-OES)
was employed to measure the amount of K, Na, Ca, and Mg loaded
There are no conflicts to declare
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DOI: 10.1039/D0CY00089B
Acknowledgements
This work was supported by the Major Research Plan of National
Natural Science Foundation of China (No. 91545114, and No.
91545203), the National Natural Science Foundations of China
on the support. CO2 temperature programmed desorption(TPD) was (No.21576227) and the 985 Program of the Chemistry and Chemical
used to measure the basicity of samples. The samples were
subjected to heat-treatment at 673 K for 1 h under flowing N2 at a
rate of 30 mL min−1 and cooled to 318 K. CO2 gas was adsorbed for
30 min. Desorption was conducted by increasing the temperature
from 318 to 1073 K at a rate of 10 °C min−1 under flowing N2at a
rate of 30 mL min−1. Diffuse reflectance ultraviolet-visible (UV-vis)
spectra of samples were recorded in the region of 200-400 nm on a
Varian Cary 5000 UV-vis spectrophotometer in Kubelka-Munk units
using BaSO4 as reference.
Engineering disciplines of Xiamen University.
References
[1]. L. Jiang; A. Zheng; Z. Zhao; F. He; H. Li; W. Liu, Bioresour
Technol, 2015, 182, 364-7.
[2]. X. L. Bai; P. Johnston; R. C. Brown, J. Anal. Appl. Pypol., 2013,
99, 130-136.
[3]. R. Vinu; L. J. Broadbelt, EnergyEnviron. Sci., 2012, 5, 9808-9826.
[4]. K. R. Brower; B. H. Davison; A. J. Ragauskas; R. Templer; T. J.
Tschaplinski; J. R. Mielenz, Science, 2006, 312, 1744-1745.
[5]. D. Santhanaraj; M. R. Rover; D. E. Resasco; R. C. Brown; S.
Crossley, ChemSusChem, 2014, 7, 3132-3137.
[6]. X. Hu; L. Wu; Y. Wang; D. Mourant; C. Lievens; R. Gunawan; C.-
Z. Li, Green Chemistry, 2012, 14, 3087.
[7]. M. Käldström; N. Kumar; T. Heikkilä; D. Y. Murzin, Applied
CatalysisA: General, 2011, 397, 13-21.
[8]. D. Liu; H. Kim Kwang; J. Sun; A. Simmons Blake; S. Singh,
ChemSusChem, 2017, 11, 598-604.
[9]. P. Mäki-Arvela; I. L. Simakova; T. Salmi; D. Y. Murzin, Chemical
Reviews, 2014, 114, 1909-1971.
[10]. M. Dusselier; P. Van Wouwe; A. Dewaele; E. Makshina; B. F.
Sels, Energy& EnvironmentalScience, 2013, 6, 1415-1442.
[11]. D. Esposito; M. Antonietti, ChemSusChem, 2013, 6, 989-
992.
[12]. W. Dong; Z. Shen; B. Peng; M. Gu; X. Zhou; B. Xiang; Y.
Zhang, Sci Rep, 2016, 6, 26713.
[13]. J. Dijkmans; M. Dusselier; D. Gabriëls; K. Houthoofd; P. C. M.
M. Magusin; S. Huang; Y. Pontikes; M. Trekels; A. Vantomme; L.
Giebeler; S. Oswald; B. F. Sels, ACS Catalysis, 2015, 5, 928-940.
[14]. P. Y. Dapsens; C. Mondelli; J. Perez-Ramirez, ChemSusChem,
2013, 6, 831-9.
[15]. P. P. Pescarmona; K. P. F. Janssen; C. Delaet; C. Stroobants;
K. Houthoofd; A. Philippaerts; C. De Jonghe; J. S. Paul; P. A. Jacobs;
B. F. Sels, Green Chemistry, 2010, 12, 1083.
[16]. W. Deng; P. Wang; B. Wang; Y. Wang; L. Yan; Y. Li; Q. Zhang;
Z. Cao; Y. Wang, Green Chemistry, 2018, 20, 735-744.
[17]. X. Lei; F.-F. Wang; C.-L. Liu; R.-Z. Yang; W.-S. Dong, Applied
CatalysisA: General, 2014, 482, 78-83.
4.4 Catalytic Evaluation
The catalytic reactions were performed in a batch-type Teflon-lined
stainless-steel autoclave reactor (50 ml). In a typical procedure, the
vessel was charged with reactants, catalysts (solid Lewis acid and
lactic acid), and deionized water, after which the autoclave was
purged five times with N2 and then pressurized to 2.0 MPa with N2
at room temperature. Next the autoclave was heated to the desired
reaction temperature and held at that temperature for required
time with stirring at 800 rpm. After a fixed time, the reaction was
quickly terminated by cooling the reactor to room temperature in
cold water. As for the conversion of glucose to methyl lactate, the
solvent is changed to methanol without extra lactic acid added.
During the recycling tests, the spent catalyst after each batch was
filtered and dried in an oven at 383K overnight, followed by
calcination in a muffle furnace for 5 hours at 823 K (5 K min-1 ramp
rate), before employed for the subsequent use.
4.5 Product analysis
Sample products were first qualitatively analyzed by an Agilent GC-
MS equipped with a 1701-ms column. The crude product was
dissolved in a silylating agent (bis-(trimethylsilyl) trifluoroacetamide
+1% trimethylchlorosilane). This solution was maintained at 65 °C
for 2 h to ensure the complete silylation which was subsequently
injected into the GC-MS. Some peaks in Agilent HPLC were further
identified using LC-MS. Quantitative analysis was then performed
on an Agilent HPLCsystem equipped with a RI and UV-Vis detectors
and a Bio-Rad Aminex HPX-87H ion exclusion column (300 × 7.8
mm), using 0.005 M H2SO4 as the mobile phase at a flow rate of 0.6
o
mL min−1. The column temperature was 50 C and the detector was
set to 50 oC. The amount of product was determined using
calibration curves generated with standard solutions. The products
collected from glucose conversion to MLA were quantitatively
analyzed by gas chromatography (model GC 2060 by Shanghai
Ruimin Instruments, Inc.) equipped with a DB-WAXETR capillary
column (50 m × 0.32 mm × 1.0 μm) and a flame ionization detector
(FID), where 1,4-dioxane was always added as an external standard.
[18]. T. Ennaert; J. Van Aelst; J. Dijkmans; R. De Clercq; W.
Schutyser; M. Dusselier; D. Verboekend; B. F. Sels, ChemSoc Rev,
2016, 45, 584-611.
[19]. H. Y. Luo; J. D. Lewis; Y. Roman-Leshkov, Annu RevChem
Biomol Eng, 2016, 7, 663-92.
[20]. P. Y. Dapsens; C. Mondelli; J. Perez-Ramirez, Chem Soc Rev,
2015, 44, 7025-43.
[21]. R. Bermejo-Deval; M. Orazov; R. Gounder; S.-J. Hwang; M. E.
Davis, ACS Catalysis, 2014, 4, 2288-2297.
[22]. M. S. Holm; S. Saravanamurugan; E. Taarning, Science, 2010,
328, 602-605.
Conflicts of interest
This journal is © The Royal Society of Chemistry 20xx
J . Name., 2013, 00, 1-3 | 7
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Please do not adjust margins
Catalysis Science & Technology
Page 8 of 9
COMMUNICATION
Journal Name
[23]. R. Otomo; R. Kosugi; Y. Kamiya; T. Tatsumi; T. Yokoi,
CatalysisScience & Technology, 2016, 6, 2787-2795.
[24]. M. Koehle; R. F. Lobo, Catalysis Science & Technology, 2016,
6, 3018-3026.
View Article Online
DOI: 10.1039/D0CY00089B
[25]. S. Tolborg; I. Sadaba; C. M. Osmundsen; P. Fristrup; M. S.
Holm; E. Taarning, ChemSusChem, 2015, 8, 613-7.
[26]. X. Yang; J. Bian; J. Huang; W. Xin; T. Lu; C. Chen; s. Yunlai; Z.
Lipeng; F. Wang; J. Xu, Fluoride-free and ultra-low concentration
template synthesis of hierarchical Sn-Beta zeolites: efficient catalyst
for conversion of glucose to alkyl lactate. 2016; Vol. 19.
[27]. B. Tang; W. Dai; G. Wu; N. Guan; L. Li; M. Hunger, ACS
Catalysis, 2014, 4, 2801-2810.
[28]. P. Li; G. Liu; H. Wu; Y. Liu; J.-g. Jiang; P. Wu, The Journal of
Physical ChemistryC, 2011, 115, 3663-3670.
[29]. A. Corma; M. E. Domine; L. Nemeth; S. Valencia, Journal of
the American Chemical Society, 2002, 124, 3194-3195.
[30]. C. Hammond; S. Conrad; I. Hermans, Angew Chem Int Ed
Engl, 2012, 51, 11736-9.
[31]. C. Hammond; D. Padovan; A. Al-Nayili; P. P. Wells; E. K.
Gibson; N. Dimitratos, ChemCatChem, 2015, 7, 3322-3331.
[32]. Y. Wan; M. Zhuang; S. Chen; W. Hu; J. Sun; J. Lin; S. Wan; Y.
Wang, ACS Catalysis, 2017, 7, 6038-6047.
[33]. F. de Clippel; M. Dusselier; R. Van Rompaey; P. Vanelderen;
J. Dijkmans; E. Makshina; L. Giebeler; S. Oswald; G. V. Baron; J. F.
Denayer; P. P. Pescarmona; P. A. Jacobs; B. F. Sels, J Am ChemSoc,
2012, 134, 10089-101.
[34]. W. Hu; Y. Wan; L. Zhu; X. Cheng; S. Wan; J. Lin; Y. Wang,
ChemSusChem, 2017, 10, 4715-4724.
[35]. C. Paris; M. Moliner; A. Corma, Green Chemistry, 2013, 15,
2101.
[36]. H. Y. Luo; L. Bui; W. R. Gunther; E. Min; Y. Román-Leshkov,
ACS Catalysis, 2012, 2, 2695-2699.
[37]. G. Martra; R. Ocule; L. Marchese; G. Centi; S. Coluccia,
CatalysisToday, 2002, 73, 83-93.
[38]. G. Tsilomelekis; M. J. Orella; Z. Lin; Z. Cheng; W. Zheng; V.
Nikolakis; D. G. Vlachos, Green Chemistry, 2016, 18, 1983-1993.
[39]. D. Padovan; C. Parsons; M. Simplicio Grasina; C. Hammond,
Green Chemistry, 2016, 18, 5041-5049.
[40]. W. N. P. van der Graaff; C. H. L. Tempelman; F. C. Hendriks;
J. Ruiz-Martinez; S. Bals; B. M. Weckhuysen; E. A. Pidko; E. J. M.
Hensen, Applied Catalysis A: General, 2018, 564, 113-122.
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