Conversion of sugars to methyl lactate with exfoliated layered stannosilicate UZAR-S4

Article abstract of DOI:10.1016/j.cattod.2020.03.064

Biomass has been shown as an alternative to fossil fuels for obtaining chemicals. In this work, the transformation of sugars into methyl lactate (ML) at 160 °C was carried out using the layered stannosilicate UZAR-S3 (University of Zaragoza-solid number 3) and the delaminated material UZAR-S4 (University of Zaragoza-solid number 4) obtained from its exfoliation. The exfoliation of UZAR-S3 to UZAR-S4 increased the accessibility of the compounds to the catalytic sites and the medium-strength acidity. Thus, the yield to ML for sucrose transformation increased from 8% for UZAR-S3 to 49.9 % for UZAR-S4. In the reusability tests, the UZAR-S4 catalyst was characterized before and after reaction by several techniques such as X-ray diffraction, thermogravimetry analysis, scanning electronic microscopy, energy dispersive X-ray spectroscopy and nitrogen adsorption. A deactivation of the catalyst was observed, which was related to carbonaceous deposits that decreased the specific surface area and the pore volume of the catalyst.

Full text of DOI:10.1016/j.cattod.2020.03.064

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Conversion of sugars to methyl lactate with exfoliated layered stannosilicate
UZAR-S4
Beatriz Murilloa, Óscar de la Iglesiaa,b, César Rubioa, Joaquín Coronasa, Carlos Télleza,*
a Instituto de Nanociencia de Aragón (INA), Instituto de Ciencia de Materiales de Aragón (ICMA) and Department of Chemical and Environmental Engineering, Universidad
de Zaragoza-CSIC, 50018, Zaragoza, Spain
b Centro Universitario de la Defensa Zaragoza, Academia General Militar, 50090, Zaragoza, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords:
Biomass has been shown as an alternative to fossil fuels for obtaining chemicals. In this work, the transformation
of sugars into methyl lactate (ML) at 160 °C was carried out using the layered stannosilicate UZAR-S3 (University
of Zaragoza-solid number 3) and the delaminated material UZAR-S4 (University of Zaragoza-solid number 4)
obtained from its exfoliation. The exfoliation of UZAR-S3 to UZAR-S4 increased the accessibility of the com-
pounds to the catalytic sites and the medium-strength acidity. Thus, the yield to ML for sucrose transformation
increased from 8% for UZAR-S3 to 49.9 % for UZAR-S4. In the reusability tests, the UZAR-S4 catalyst was
characterized before and after reaction by several techniques such as X-ray diffraction, thermogravimetry
analysis, scanning electronic microscopy, energy dispersive X-ray spectroscopy and nitrogen adsorption. A de-
activation of the catalyst was observed, which was related to carbonaceous deposits that decreased the specific
surface area and the pore volume of the catalyst.
Heterogeneous catalysis
Sugar conversion
Lactic acid
Layered stannosilicate
Exfoliated material
1. Introduction
alkyl lactate, distillation and subsequent hydrolysis, which suppose a
high energy cost [5].
In the last decades the transformation of renewable biomass to
generate energy or added value products has become a field of growing
interest for many researchers because of uncertainty about fossil re-
sources and environmental concerns. Among the variety of products
that can be obtained from biomass, lactic acid is of significant im-
portance due to its ability to serve as a platform chemical for a wide
range of applications in the chemical, pharmaceutical, cosmetics and
food industries [1]. Beyond its traditional uses, currently there is an
increased demand for lactic acid as a feedstock for the production of
biopolymer poly-lactic acid (PLA), which is a promising biodegradable,
biocompatible, and environmentally friendly alternative to plastics
derived from petrochemicals [2]. PLA can be used to produce com-
mercial products related to packaging, agricultural uses, transportation,
electronics and houseware, among others [3].
The main way for the production of lactic acid is by means of the
fermentation of carbohydrates (usually pentoses or hexoses) in aqueous
solutions. Nevertheless, nutrient costs, gypsum waste produced in the
neutralization step and low volumetric productivities are major draw-
backs of this process [4]. Furthermore, the purification of lactic acid
from aqueous solutions is a complex process since it involves the es-
terification of lactic acid with an alcohol to obtain the corresponding
As an alternative to fermentation, catalytic conversion of carbohy-
drates for the production of lactic acid arose as process worth studying.
In fact, first with homogeneous catalysts, Hayashi et al. reported the
pioneering catalytic conversion of trioses to lactic acid with a Sn halide
[6]. Since then, the use of other homogeneous catalysts have been re-
ported such as SnCl4 [7] or Sn4+-based organometallic complexes [8].
On the other hand, the use of a heterogeneous catalytic process
facilitates the recovery of the catalyst after reaction and its later reuse.
In this context, Janssen et al. [9], used various FAU-type zeolites in
ethanol for the conversion of trioses reaching a yield to ethyl lactate up
to 65 % after 6 h at 90 °C. Other Sn-based catalysts were applied to the
conversion of trioses to lactic acid or alkyl lactates, for instance, Sn-MFI
[10], Sn-MCM-41 [10–12], Sn-SBA-15 [10], Sn-MWW [13], Sn-Si mixed
oxides [14] or recently Sn-Nb mixed oxides [15], the last catalysts with
methyl lactate yield of 98 % after 2 h at 140 °C. Further, other metal
oxide without tin (e.g. γ-Al2O3 [16] or Nb2O5 [17]) or metal phosphates
(e.g. with Sn [18] or Nb [19]) have been tested for the conversion of
dihydroxyacetone to lactic acid derivates. In last catalysts using tin
phosphates, Wang et al. [18] have reached yield of 96 % of lactic acid
after 4 h at 140 °C.
While the conversion of trioses to lactic acid or alkyl lactates
⁎ Corresponding author.
E-mail address: ctellez@unizar.es (C. Téllez).
https://doi.org/10.1016/j.cattod.2020.03.064
Received 18 November 2019; Received in revised form 4 March 2020; Accepted 27 March 2020
0920-5861/©2020ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:BeatrizMurillo,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.03.064

B. Murillo, et al.
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provides a solid test for benchmarking catalysts, the use of abundant
hexoses (glucose or fructose) or even di- or polysaccharides (sucrose) is
industrially preferred [20]. To carry out these reactions, higher reaction
temperatures are required (≥ 150 °C) in comparison with those applied
to trioses (usually 90 °C). This results in more side reactions, and
consequently lower yields to lactic acid or alkyl lactates are obtained
[4]. Holm et al. [21] reported the conversion of sucrose to alkyl lactates
using zeolites Ti-Beta, Zr-Beta and Sn-Beta with a maximum yield of ML
of 64 % at 160 °C for 20 h. Recently, other authors have reported the
utilization of a carbon-silica composite [11], Sn-MWW-type zeolite
[13], hierarchical zeolite Sn-Beta [22], Fe-doped SnO2 [23], Mg-MOF-
74 [24] or γ-NiOOH [25], among others. In previous works, we also
applied several metal organic frameworks: Zn imidazolate ZIF-8 [26]
and Sn-based carboxylates UZAR-S10 and MIP-177-LT (Ti/Sn) [27] for
this reaction with good results.
Layered silicates represent a very attractive field of research because
of the advantages of layered materials as active catalysts [28]. A
characteristic feature of some layered materials is their ability to
swelling and delamination, processes that involve the intercalation of
guest molecules to increase the interlayer space and subsequent ex-
foliation.
In a previous work, we reported the use of delaminated materials
UZAR-S1 and UZAR-S2 in the catalytic conversion of glucose with an
increase of methyl lactate yield with respect to their precursor mate-
rials, microporous titanosilicates JDF-L1 and AM-4, respectively, since
they provided a better access of the reactants to catalytic sites, thus
increasing the catalytic activity of the materials but with low methyl
lactate yields, below 6% [29].
2.2. Catalyst characterization
Phase identification was carried out by a D-Max 2500 Rigaku X-ray
diffractometer with a copper anode and a graphite monochromator
using Cu-Kα1 radiation (λ =1.5418 Å), taking data from 2θ = 5° to 40°
at a scan rate of 0.03°s−1 and operating parameters of 40 kV and
80 mA.
Nitrogen adsorption–desorption isotherms were obtained with a
Micromeritics Tristar 3000 at -196 °C. Before these measurements, the
samples were degassed at 200 °C, during 8 h under vacuum. The specific
surface area was calculated using the Brunauer-Emmett-Teller (BET)
equation.
Thermal behavior was determined by thermogravimetric analysis
(TGA) which was carried out using a Mettler Toledo TGA/SDTA 851e
system. Samples (about 5 mg) were placed in 70 mL alumina pans and
then heated under air flow up to 800 °C at a heating rate of 10 °C·min−1
.
Finally, scanning electronic microscopy (SEM) and energy dispersive X-
ray spectroscopy (EDX) were performed over Pt-coated specimens using
an Inspect F50 model scanning electron microscope (FEI).
Pyridine FTIR was used to evidence the presence of Brønsted and
Lewis acidic sites. UZAR-S3 and UZAR-S4, previously dried at 100 °C
overnight, were exposed for adsorption to the pyridine vapor for 24 h at
ambient temperature. After this adsorption step, the catalyst samples
were exposed to a nitrogen stream of 50 m L(STP)· min−1 at 80 °C for
30 min to remove the physisorbed pyridine. FTIR spectra were achieved
with a Vertex 70 of Bruker and a Specac’s Golden Gate ATR.
Temperature-programmed desorption studies of NH3 (NH3-TPD)
were performed for UZAR-S3 and UZAR-S4. These analyzes have been
carried out in the Micromeritics AUTOCHEM II 2920 unit with thermal
conductivity detector. The test conditions have been as follows. First,
samples were dried in a He stream of 50 m L(STP)· min−1 up to 200 °C
at 10 °C min−1 and an isothermal time of 60 min. After cooling the
samples to 100 °C, in He 50 m L(STP)·min−1, the adsorption step was
carried out with a stream of 50 m L(STP)· min−1 of 0.5 % NH3/He at
100 °C for 1 h. For the correct desorption of the physisorbed adsorbate,
a He stream of 50 m L(STP)· min−1 was passed through at 100 °C for 1 h.
Finally, desorption of the adsorbate was analyzed with a He stream of
In this work the delaminated stannosilicate UZAR-S4 was applied as
catalyst to the conversion of glucose and sucrose to methyl lactate. Tin
containing catalysts have been shown in the literature as active and
selective for this reaction [10,27]. USAR-S4, that was previously syn-
thesized in our group [30], is obtained from the exfoliation of USAR-S3
which consists of SiO4 tetrahedral units observed by 29Si NMR and
octahedra SnO6 observed by 119Sn NMR, therefore these materials be-
long to the so-called OPT family (Octahedral-Pentahedral-Tetrahedral
(OPT) framework silicates) [31].
50 m L(STP)· min−1 upon heating up to 700 °C at 10 °C·min−1
.
2. Materials and methods
2.3. Catalytic tests
2.1. Synthesis of catalysts
Conversion of sugars to methyl lactate was performed in 35 m L-
Teflon-lined autoclave. For the catalytic tests 225 mg of D-(+)-glucose
(99 wt%, Alfa Aesar) or sucrose (99 wt%, Sigma-Aldrich), 8.0 g of me-
thanol (Multisolvent HPLC grade, Scharlau), 160 mg of catalyst and
30 mg of naphthalene (99 wt%, Sigma-Aldrich), as internal standard,
were added to the autoclave. Then, the mixture was heated to 160 °C for
20 h in a rotary oven at 15 rpm. Finished the reaction time, the catalyst
was recovered by centrifugation and dried in an oven at 70 °C overnight
prior to its reutilization.
The determination of products in the reaction liquid was carried out
in the Gas Chromatograph (Agilent 6850) equipped with a capillary
column HP-5MS (30 m×0.250 mm × 0.25 μm). The equipment is
coupled with an Agilent 5975C mass spectrometry detector. The
amount of different compounds was determined based on the internal
standard of naphthalene using the calibration curves made with the
commercial compounds methyl-S-(-)-lactate (ML, 98 wt%, Sigma-
Aldrich), methyl glycolate (MG, 98 wt%, Alfa Aesar), pyruvaldehyde
dimethyl acetal (PADA, 97 wt%, Sigma-Aldrich), and 1,1,2,2-tetra-
methoxypropane (TMP, 99 wt%, Sigma-Aldrich). Unidentified com-
pounds detectable by GCeMS were calculated from the response factor
of the methyl lactate, while the reaction yields were calculated on a
carbon basis. On the other hand, sugars were determined using a
commercial analytical method (sucrose/fructose/D-glucose Assay Kit,
Megazyme). The concentration of sugar is related with that of reduced
nicotinamide adenine dinucleotide phosphate (NADPH), which has a
UZAR-S4 was prepared as reported elsewhere [30] by delamination
of stannosilicate UZAR-S3. For the synthesis of UZAR-S3 a sodium si-
licate solution (27 wt% SiO2, 8 wt% Na2O, Merck) was mixed with
deionized water and NaOH (98 wt%, Sigma-Aldrich), afterwards tin(II)
chloride dihydrate (reagent grade 98 wt%, Sigma-Aldrich) was added.
The molar composition of this gel was: 4.2 Na2O:1 SnCl2:2.9 SiO2:101
H2O. The gel was stirred for 1.5 h at room temperature and degassed for
a few minutes in an ultrasound bath. The crystallization was carried out
in a 40 mL Teflon-lined autoclave under hydrothermal conditions at
230 °C for 96 h. The obtained powder was filtered, washed repeatedly
with deionized water and dried at 100 °C overnight.
UZAR-S3 was proton exchanged and subsequently swollen by the
intercalation of nonylamine molecules. For the proton exchange, the
powder was immersed in a solution of water and acetic acid (glacial
acetic acid in deionized water, pH = 2.7) for 10 min, the pH reaching a
value of 3.9. Then, a solution of nonylamine was added and the reaction
was carried out under reflux at 60 °C for 14 h at a constant pH 9.6.
Swollen UZAR-S3 was recovered by centrifugation and dried at 70 °C
for 3 h.
Finally, UZAR-S4 was obtained by extracting the amine with an
HCl/H2O/ethanol solution (5:15:870, molar ratio) under reflux at 55°C
for 8 h. After this time, the solid was recovered, washed 5 times with
deionized water and dried at 100 °C for 12 h. More details of the
synthesis of catalysts can be found in a previous publication [30].
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Fig. 2. NH3 TPD of UZAR-S3 and UZAR-S4.
Fig. 1. FTIR spectra of UZAR-S3 and UZAR-S4 as made, and UZAR-S3 and
UZAR-S4 after pyridine adsorption and heating at 353 K.
silanol groups. This occurs in the study of some layered zeolites and
silicates, where the limitations to allow pyridine access [34] are saved
upon delamination, in agreement with the change of textural properties
(increase of external surface area) an increase of silanols (as typically
observed by solid state NMR [30]). Therefore, the TPD analysis with
ammonia has been performed due to the fact that ammonia is another
relatively strong basic molecule commonly used to probe acid sites in
catalysts like zeolites. Ammonia, with a smaller kinetic diameter, 3.0 Å,
than pyridine, has a greater accessibility to the pores.
specific absorbance band at 340 nm. The variation of absorbance was
measured using a V-670 Jasco UV–vis spectrophotometer.
3. Results and discussion
3.1. Characterization of catalyst
The total acidity of UZAR-S3 and UZAR-S4 catalysts, as well as the
strength of these acid sites, can be compared using the ammonia TPD
(Fig. 2). Like other authors [23], it has been considered that peaks
below 200 °C, between 200 and 400 °C and above 400 °C correspond to
weak, medium and strong acid sites, respectively. The total value of
ammonia in UZAR-S3 is 1.45 mmol· g−1 while in UZAR-S4 it is
2.58 mmol· g−1. These values are consistent with what was previously
seen with pyridine FTIR and UZAR-S4 show a greater amount of ac-
cessible acid sites than UZAR-S3. Moreover, the desorption tempera-
tures observed (Fig. 2) for UZAR-S4 are 160 °C (weak), 283 °C
(medium), 424 °C (strong) and 496 °C (strong), whereas UZAR-S3 pre-
sents them at 227 °C (medium), 375 °C (medium) and 464 °C (strong). In
Fig. 2 the increase in acidity of UZAR-S4 is observed, which is related to
the appearance of weak acidity and above all the increase in medium
acidity.
The preparation and characterization of materials UZAR-S3 and
UZAR-S4 were previously reported [30]. In this work, the same pre-
paration procedure was reproduced: first the synthesis of the layered
material UZAR-S3, after its swelling with nonylamine to obtain swollen
UZAR-S3 and finally the delamination of the latter by extraction with
hydrochloric acid in a mixture of solvents (ethanol and water) to obtain
UZAR-S4. Only the concentration of ethanol in the extraction solution
(from HCl/H2O/ethanol 5:17:870 to 5:15:870 here) has been slightly
varied to favor the nonylamine extraction and therefore the process of
delamination. With this slight variation, the average specific surface
value of the three prepared samples of UZAR-S4 is 352
38 m2·g−1
,
slightly higher than the highest value reported in [30] of 269 m2·g−1
.
In order to understand the acidity of UZAR-S3 and UZAR-S4, pyr-
idine FTIR and NH3-TPD analyzes were performed. Fig. 1 shows the
FTIR spectra before and after pyridine adsorption from UZAR-S3 and
UZAR-S4.
3.2. Catalytic results
In the case of UZAR-S4, several peaks are appreciated after pyridine
adsorption. The peak at 1618 cm−1 is due to strong Lewis acid sites (SL)
[23] while peaks at 1577 cm−1 and 1559 cm−1 correspond to weak
Lewis acid sites (WL). The band centered at 1490 cm−1 indicates that
pyridine may be coordinated with Lewis acid sites, while protonated
pyridine would be interacting with Brønsted acid sites (B + L) [32]. At
1598 cm−1 there is a peak due to the hydrogen bond interaction be-
tween pyridine and the catalyst surface (H) [23]. In addition, there are
two more peaks that correspond to the Brønsted acid sites (B) at
1637 cm−1 and 1541 cm−1 [32]. The presence of both Brønsted and
Lewis acid sites, coming, respectively, from silanol groups and metal
vacant orbitals, would make UZAR-S4 suitable for obtaining methyl
lactate as observed by other authors in their catalysts [23].
On the other hand, no significant bands of adsorbed pyridine are
observed in UZAR-S3. This may be due to the fact that: i) pyridine, with
a kinetic diameter of 5.7 Å [33], cannot penetrate into the UZAR-S3
pores (nitrogen cannot go inside the pores) because it is not delami-
nated and does not have access, ii) the delaminated material UZAR-S4
is rich in high aspect ratio particles with a higher concentration of
The results obtained in the conversion of sugars (glucose and su-
crose) using UZAR-S3 and UZAR-S4 as catalysts for 20 h at 160 °C are
shown in Table 1 and Fig. 3. In the case of glucose (Fig. 3), the yield to
methyl lactate increased when the catalyst used in the reaction was
swollen and later delaminated: 3.2 % with UZAR-S3, 12.2 % with
swollen UZAR-S3 and 37.4 % with UZAR-S4.
This suggests that the delamination of UZAR-S3 started with the
intercalation of the amine, advancing part of the catalytic properties of
the fully delaminated UZAR-S4. In addition, glucose conversion also
increased from 89 % to 98 % (Table 1). This is due to the fact that
delamination provides a good access of the reactants and intermediate
products to catalytic sites as well as a fast desorption of products, both
effects result in an increase of the catalytic activity of the material. In
addition, as seen above, the catalyst UZAR-S4 has a higher proportion
of acid sites, (especially medium-strength acid sites) than UZAR-S3 and
these acid sites become, through delamination, accessible for large
molecules. This is in agreement with the literature where the combi-
nation of Lewis and Brønsted acids along with the medium-strength
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Table 1
Catalytic results obtained for the conversion of sugars (160 °C, 20 h, 160 mg of catalyst and 225 mg of sugar) with different catalysts. Methyl lactate (ML), methyl
glycolate (MG), pyruvaldehyde dimethyl acetal (PADA), 1,1,2,2-tetramethoxypropane (TMP) and non-identified products (n.i.p.). Mean values and the corresponding
standard deviations were calculated from 4 replicated experiments in the same operation conditions.
Catalyst
Sugar
Yield (%)
ML
Total
yield (%)
Sugar
conv. (%)
MG
PADA
TMP
n.i.p.
Blank
UZAR-S3
Swollen UZAR-S3
UZAR-S4
Blank
UZAR-S3
Swollen UZAR-S3
UZAR-S4
Glucose
Glucose
Glucose
Glucose
Sucrose
Sucrose
Sucrose
Sucrose
1.1
3.2
12.2
37.4
1.4
8.0
33.0
49.9
0.7
1.3
2.5
3.6
0.9
0.6
3.1
9.6
–
2.5
1.1
3.3
6.2
2.5
0.5
1.7
5.3
1.2
0.5
0.9
1.3
1.4
0.1
0.1
1.1
–
6.0
5.6
9.2
7.8
4.3
3.8
6.0
6.7
1.7
1.2
1.0
1.5
1.3
0.3
1.6
1.6
9.6
87
89
4.3
2.9
1.5
–
1.2
0.4
0.1
1.2
0.9
0.6
0.5
1.5
5.6
0.1
0.2
0.6
1.5
0.2
0.6
1.8
0.04
0.07
0.2
14.7
29.1
58.5
8.3
13.7
41.7
63.5
89.6
98
88.6
88.8
89.5
99.5
0.2
0.2
0.01
0.4
sucrose was used instead of glucose. The swollen material shows a
greater increase, from 12.2% to 33.0%, due to the above mentioned
incipient delamination. And delaminated material UZAR-S4 increased
the yield from 37.4% to 49.9%. This is because sucrose is a non-redu-
cing sugar, which has an o-glycosidic bond that protects carbonyl
groups. In this way, the release of hexoses from sucrose in solution is
slow, avoiding the occurrence of unwanted side reactions with the more
reactive glucose and fructose, and a less amount of by-products is
generated [26,37]. Regarding the sugar conversion, this was similar for
glucose and sucrose, being almost total with delaminated UZAR-S4.
The amount of catalyst used in the reaction was studied, keeping
constant the amount of the other reagents (225 mg of sucrose and 8 g of
methanol). Different experiments were performed with 50, 80, 100 and
160 mg of catalyst. Average methyl lactate yields and standard devia-
tions are shown in Fig. 4. It can be seen that the difference in ML yield is
negligible using 100 mg instead of 160 mg of catalyst. Nevertheless, the
ML yield decreased if the amount of catalyst was reduced to 80 or
50 mg. Therefore, 100 mg is considered to be the optimum amount of
catalyst, because the preparation of the material is expensive and it is
desirable to use the smallest amount of it, while maintaining a high
level of catalytic activity. With a reduction in the amount of catalyst
near 40 % the ML yield was reduced only a 3.3 %.
Fig. 3. ML yield obtained for the conversion of sugars (160 °C, 20 h, 160 mg of
catalyst and 225 mg of sugar) with different catalysts.
acid sites favors the conversion to methyl lactate [23]. Then, except for
MG, the yield to other products slightly increased with the delaminated
catalysts (Table 1). It should be taken into account that glucose has
several reaction pathways [29], one of them produces during the retro-
aldol condensation erythrose and glycolaldehyde. Then, glycolaldehyde
can evolve into the identified MG, and erythrose can evolve into methyl
vinyl glycolate [11,35]. This last compound could be related to some of
the unidentified peaks. The another pathway forms compounds with
three carbon atoms (glyceraldehyde and dihydroxyacetone) that finally
can be transformed into ML, PADA and TMP. Therefore, the delami-
nated catalyst seems to favor this second pathway but not the first.
It should also be commented that since the steric hindrances in
UZAR-S4 are lower than in UZAR-S3, the formation of several bigger
compounds through the kind of reactions studied here is plausible. In
fact, it has been observed that the pore size in mesoporous materials
(e.g. MCM-41) in comparison with materials with smaller pores (e.g.
zeolite beta) can do shape-selective control [36]. However, these
compounds was not detected here, although there was an increase in
unidentified compounds, which could be related to them.
3.3. Comparison with literature
The yield to methyl lactate achieved using UZAR-S4 as catalyst was
37.4 % with glucose and 49.9 % with sucrose. These values are in the
order of those obtained by Holm et al. [21] with Sn-beta zeolite as
The same increase in ML yield was reported in the conversion of
glucose with delaminated porous titanosilicates as catalysts [29]: 3.9 %
with UZAR-S1 and 1.8 % with its precursor JDF-L1; and 5.2 % with
UZAR-S2 and 1.2 % with its precursor AM-4. The much better results of
UZAR-S4 can be attributed mainly to the presence of tin in the struc-
ture, that has a higher catalytic activity for this reaction than titanium
[29].
Regarding the catalytic tests carried out with sucrose. In all cases,
the yields to methyl lactate are better using sucrose instead of glucose
(Fig. 3). In the case of UZAR-S3 it increases from 3.2% to 8.0%, if
Fig. 4. ML yield in the conversion of sucrose with different amounts of UZAR-
S4 as catalyst. Experimental conditions: 160 °C, 20 h and 225 mg of sucrose.
Error bars correspond to standard deviations from at least 3 replicated runs.
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Table 2
Literature review of methyl lactate yield from sucrose under experimental conditions analogous to those applied in this work using heterogeneous catalysts.
Catalysts
T (ºC)
t (h)
MeOH (g)
Sucrose (mg)
Catalysts (mg)
ML Yield
Ref
Ti-Beta
Zr-Beta
Sn-Beta
160
160
160
160
155
160
170
170
170
170
170
160
160
160
160
220
160
200
160
20
20
20
16
20
20
16
16
16
16
16
5
24
20
20
6
24
12
20
8
8
8
15
6
225
225
225
450
225
112.5
450
450
450
450
450
320
225
225
220
60
160
160
160
150
160
80
150
150
150
150
150
200
160
160
160
20
44
40
64
59
45
55
59
52
66
67
62
60
34.8
72.1
44
[21]
[21]
[21]
[10]
[11]
[13]
[38]
[38]
[38]
[38]
[38]
[39]
[26]
[22]
[23]
[24]
[27]
[25]
This work
Sn-Beta
CSM(1)
Sn‐MWW
4
Li-Sn-Beta
Na-Sn-Beta
K-Sn-Beta
Rb-Sn-Beta
Cs-Sn-Beta
Sn-Beta-WO3
ZIF-8
15
15
15
15
15
12
8
8
16
7
Hierarchical Sn-Beta
Fe-doped SnO2
Mg-MOF-74
MIP‐177‐LT (Ti/Sn)
γ-NiOOH
47
8
40
8
225
400
225
160
200
160
39.4
38.7
49.9
UZAR-S4
(1) CSM: Carbon silica composite material.
catalyst: 44 % for glucose and 64 % for sucrose and the yield value
reached with Sn-MCM-41 43 % for glucose [29]. Table 2 shows a re-
view of heterogeneous catalysts with featured results in the conversion
of sucrose to methyl lactate. This table includes results with reaction
conditions similar to those tested here.. The performance of this work is
among the best in the literature, the zeolite Sn-beta based catalysts
being those that have achieved the best results to date. Specifically, the
highest yield corresponds to a Sn-beta zeolite with hierarchical pore
structure that favors the access and desorption of compounds in the
catalytic sites, as occurs as a result of the exfoliation strategy followed
here.
activity, the yield to ML decreasing by up to 30.5 %. Therefore, the
physical-chemical changes that occurred in the catalyst during its use
were studied to find out the reasons for this deactivation. The other
products remained practically constant during the successive catalytic
cycles performed.
3.5. Characterization of catalyst after catalytic cycles
The X-ray diffractograms of UZAR-S4 before and after one catalytic
test with sucrose are shown in Fig. 6a. As can be observed, UZAR-S4 has
the XRD features corresponding to an amorphous material and no ap-
preciable changes occurred during the reaction. It should be noted that
other porous delaminated materials also show an amorphous X-ray
diffractogram [40], but in the specific case of the UZAR-S4 material the
electron diffraction performed from some crystallites suggests that the
structure would be maintained at some extension respect to the UZAR-
S3 material [30].
In the thermogravimetric analysis performed and shown in Fig. 6b,
it can be seen that the weight loss curve of fresh UZAR-S4 is clearly
different from that of the recovered solid after reaction. UZAR-S4 before
the reaction has a main loss below 100 °C due to adsorbed solvents
(water and ethanol), while in the case of UZAR-S4 after the reaction the
curve present several weight losses above 100 °C due to adsorption of
carbonaceous compounds on the surface of UZAR-S4 during the reac-
tion.
3.4. Reusability of UZAR-S4
The interest of a heterogeneous catalyst is in its reusability, there-
fore the UZAR-S4 solid was used in three consecutive catalytic cycles of
sucrose transformation using 100 mg of catalysts. Between the catalytic
cycles, the catalyst was dried in the oven at 150 °C for 12 h in order to
remove weakly adsorbed compounds.
The yield to different products during 3 catalytic cycles is shown in
Fig. 5. It can be seen that when reusing UZAR-S4 catalyst in a second
catalytic cycle the ML yield slightly decreased by only 4% (from 46.6%
to 42.6%). However, a third catalytic cycle implied a loss of catalytic
Fig. 7 shows the SEM images of UZAR-S4 before and after reaction
with sucrose. The external morphology of UZAR-S4 is evident in Fig. 7a
of sheets with a thickness of ca. 100 nm. After reaction, in general, there
were no substantial changes in the appearance of the particles although
some particles seem to have been fragmented.
In addition to SEM, energy dispersive X-ray spectroscopy (EDX) was
performed to the same samples. Si/Sn atomic ratio of fresh UZAR-S4
was 39
sucrose was 42
17 and Si/Sn ratio of the solid recovered after reaction with
16. Variations may indicate leaching of tin in the
reaction solution but the variation is in the error and cannot be affirmed
emphatically. This leachate could be the cause of the loss of catalytic
activity but as will be seen below there were more relevant textural
changes in the catalyst during reaction. Given the high methyl lactate
yield, the contribution of catalytic activity in homogeneous phase of
this possible low amount of tin in the solution would have low influence
on the catalytic results.
Fig. 5. Yield of different products in the conversion of sucrose with UZAR-S4 up
to 3 catalytic cycles. Experiment conditions: 160 °C, 20 h, 100 mg of catalyst
and 225 mg of sugar.
Table 3 shows the textural properties of UZAR-S4 material before
and after the reaction with sucrose for each catalytic cycle. There is a
5

B. Murillo, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 6. UZAR-S4 before and after its use in one catalytic test with sucrose (160 °C, 20 h, 100 mg of catalyst and 225 mg of sugar): a) X-ray diffractograms. b) TGA
curves.
slight decrease in the BET area after the first cycle (from 352 to
315 m2 ·g−1) and also in the pore volume (from 0.25 to 0.22 cm3 ·g−1).
This fact could be attributed to the compounds adsorbed in the catalyst
(in line with the TGA curves in Fig. 6b), a deterioration of catalyst
structure and a change in its chemical composition due to leaching.
Degasification prior to the measure of nitrogen isotherms was carried
out at 200 °C, nevertheless in the TGA curve of the recovered solid after
reaction with sucrose there is a weight loss of 13.5 % from 200 °C. This
suggests that certain adsorbed compounds (reaction products) could
decrease both surface area and pore volume. After the second and third
cycles, the decrease in the specific surface area and pore volume be-
came greater, indicating a further degradation of the material UZAR-S4
with the continued use. This loss of textural properties after the second
cycle is in agreement with the decrease in ML yield (see Fig. 5).
Table 3
Textural properties of UZAR-S4 fresh and after each catalytic cycle. Pore vo-
lume measured at P/P0 = 0.97.
Sample
BET specific surface area [m2 g−1
]
Pore volume [cm3 g−1
]
Fresh UZAR-S4
After cycle 1
After cycle 2
After cycle 3
352
315
174
89
38
15
16
0.35
0.29
0.22
0.12
5
addition, the catalytic acid sites have the appropriate combination of
Lewis and Brønsted acid sites and in UZAR-S4 the medium-strength acid
sites are greater compared to those in UZAR-S3. Regarding its stability,
UZAR-S4 was reused up to three times, with a reduction of methyl
lactate yield that can be explained by the degradation of the material in
the reaction medium in agreement with the decrease of specific surface
area. UZAR-S4 can be considered an interesting material in the con-
version of sugars since the results obtained are in the order or even
better than those reported before with other heterogeneous catalysts.
4. Conclusions
Stannosilicate UZAR-S4 was successfully synthesized by delamina-
tion of UZAR-S3. This material can be applied as catalyst for the con-
version of glucose and sucrose with high activity and selectivity to
methyl lactate. The higher ML yield was 49.9 % in the conversion of
sucrose at 160 °C for 20 h with 160 mg of catalyst with a conversion of
sugar near 100 %. Furthermore, the delamination of the catalyst in-
creases its activity (ML yield increased from 8.0 % for UZAR-S3 to 49.9
% with UZAR-S4 in the conversion of sucrose) because it improves the
access of reactants and intermediate products to catalytic sites. In
CRediT authorship contribution statement
Beatriz Murillo: Investigation, Methodology, Formal analysis,
Writing
Methodology, Formal analysis, Writing - original draft, Writing - review
& editing, Visualization. César Rubio: Investigation, Methodology,
- original draft. Óscar de la Iglesia: Investigation,
Fig. 7. SEM images of UZAR-S4: a) before reaction; b) after reaction with sucrose (160 °C, 20 h, 100 mg of catalyst and 225 mg of sugar).
6

B. Murillo, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Formal analysis, Writing - original draft, Writing - review & editing.
Joaquín Coronas: Conceptualization, Methodology, Supervision,
Project administration, Funding acquisition, Writing - original draft,
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
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