Synthesis and Characterization of Sn, Ge, and Zr Isomorphous Substituted MFI Nanosheets for Glucose Isomerization to Fructose

Article abstract of DOI:10.1002/cplu.202100289

Various metals including Sn, Ge, and Zr have been successfully incorporated into the MFI nanosheets via a one-pot synthesis. The as-synthesized zeolites exhibit high external surface area and mesopore volume without large metal oxides aggregated on zeolite surfaces. Interestingly, the successful introduction of heteroatoms in MFI nanosheets can be confirmed by shifted XRD peaks corresponding to the unit cell expansion due to the replacement of metals into the framework. In addition, the UV-Vis absorbance spectra reveal that at the suitable metal loading the incorporated tetrahedral coordination of metal species in the zeolite framework has been obtained. To illustrate the benefits of the prepared catalysts, the glucose isomerization to fructose was carried out in a water/dioxane system. Obviously, the SnMFI-NS samples, containing the high dispersion of metal isomorphous species demonstrate the outstanding catalytic behavior in term of fructose selectivity (>85 %).

Full text of DOI:10.1002/cplu.202100289

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Synthesis and Characterization of Sn, Ge, and Zr
Isomorphous Substituted MFI Nanosheets for Glucose
Isomerization to Fructose
Pannida Dugkhuntod, Narasiri Maineawklang, Chadatip Rodaum, Peerapol Pornsetmetakul,
[a]
Kachaporn Saenluang, Saros Salakhum, and Chularat Wattanakit*
Various metals including Sn, Ge, and Zr have been successfully
incorporated into the MFI nanosheets via a one-pot synthesis.
The as-synthesized zeolites exhibit high external surface area
and mesopore volume without large metal oxides aggregated
on zeolite surfaces. Interestingly, the successful introduction of
heteroatoms in MFI nanosheets can be confirmed by shifted
XRD peaks corresponding to the unit cell expansion due to the
replacement of metals into the framework. In addition, the UV-
Vis absorbance spectra reveal that at the suitable metal loading
the incorporated tetrahedral coordination of metal species in
the zeolite framework has been obtained. To illustrate the
benefits of the prepared catalysts, the glucose isomerization to
fructose was carried out in a water/dioxane system. Obviously,
the SnMFI-NS samples, containing the high dispersion of metal
isomorphous species demonstrate the outstanding catalytic
behavior in term of fructose selectivity (>85%).
Introduction
portation and reduce the coke deposition, resulting in an
improved catalytic lifetime.
[5]
Zeolites are microporous crystalline aluminosilicate materials
demonstrating many outstanding properties, in particular high
thermal/hydrothermal stability and adjustable acidity, eventu-
Moreover, zeolite catalysts have been modified with various
metals to adjust the appropriate acid strength and active sites.
In general, some metals, such as iron (Fe), zinc (Zn), and gallium
(Ga) were loaded on zeolites obtained via simple ion-exchange
[1]
ally resulting in using them as solid supports and catalysts.
[
6]
Among them, the MFI is one of the most promising zeolite
types, which has been extensively utilized in different catalytic
applications in petrochemical industries because of their wide-
range catalytic properties and their appropriate medium porous
structures of 10 membered rings, which are suitable for various
and impregnation methods to reduce their acid strength.
However, the metal species obtained from each loading
method are different and appropriate for different catalytic
reactions. In addition, another alternative approach to incorpo-
rate the metal in a zeolite is the isomorphous substitution of
zeolite frameworks with tetravalent metal ions, such as
germanium (Ge), tin (Sn), titanium (Ti), and zirconium (Zr),
[2]
chemical reactions.
However, a conventional zeolite, containing a sole micro-
porous structure, exhibits some drawbacks with regard to the
diffusion limitation inside the zeolite network. This results in the
pore-blockage, and therefore bulky molecules cannot penetrate
easily to active sites located in micropores, eventually leading
to the fast deactivation of catalysts. To overcome these
problems, over the past decade, the hierarchical zeolites with
additional mesopores and/or macropores have been developed
[7]
which can substitute into the zeolite framework. For example,
these metals can act as Lewis acid sites demonstrating the acid
strength in the order of Si!Ge<Ti<Sn�Zr, and they are
appropriate for various catalytic applications, especially in
[8]
biomass conversion.
Indeed, over the past decades, researchers have studied
the characteristic properties of the heteroatom-substituted
zeolite frameworks to gain insights into their structural
[3]
intensively. Typically, they have been synthesized via both
post-synthesis methods (desilication and dealumination) and
[
9]
analysis and their catalytic behavior. For example, the
germanium (Ge), which has similar properties to silicon, is one
of the most interesting elements, which has been widely used
[4]
templating methods. It was confirmed that the additional
mesopores and/or macropores facilitate the molecular trans-
[
7b,10]
to design metal-isomorphously substituted zeolites.
The
conventional Ge-ZSM-5 was successfully synthesized by direct
synthesis, and XAS results could confirm the presence of
tetrahedral Ge in the zeolite framework. In addition, other
metals such as zirconium (Zr) and tin (Sn) incorporated in
zeolites, well-known active sites for biomass conversion, such
[
a] P. Dugkhuntod, N. Maineawklang, C. Rodaum, P. Pornsetmetakul,
K. Saenluang, Dr. S. Salakhum, Prof. Dr. C. Wattanakit
Department of Chemical and Biomolecular Engineering
School of Energy Science and Engineering
Nanocatalysts and Nanomaterials for Sustainable Energy
and Environment Research Network of NANOTEC
Vidyasirimedhi Institute of Science and Technology
Rayong 21210 (Thailand)
[
7b]
[
11]
[12]
[13]
as
isomerization,
dehydration,
hydrogenation,
[
14]
[15]
[16]
hydroxylation, oxidation, and epoxidation, have been
studied for some catalytic applications. For example, Song
Song, et al. described the incorporation of Zr in a mesoporous
Beta zeolite framework for the conversion of biomass-derived
E-mail: chularat.w@vistec.ac.th
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.202100289
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furfural to γ-valerolactone. Its hierarchically micro-mesoporous
strates the related synthesis procedure in which the quaternary
ammonium cation, TBAOH, was applied as a hierarchical
structure-directing agent (SDA) to direct both the microporous
structure and the self-assembly of zeolite nanosheets in the
presence of metal precursors. To confirm the crystalline
structure, the XRD patterns of all the prepared catalysts with
different metal contents exhibit high crystallinity with the
characteristic peaks of MFI framework appeared at 2θ of 7.8,
8.7, 23.1, and 23.8°, attributing to the plane indices of (101),
structure is crucial to facilitate molecular transport in a zeolite
[
17]
network. Moreover, hierarchical Sn-MFI nanosheets with the
different Si/Sn ratio of 75, 186, and 223 have been manufac-
tured by a one-pot synthesis. The designed materials could
provide the high yield of fructose and lactulose obtained from
[
9b]
glucose and lactose conversion, respectively. Although there
are some related literatures describing about the development
of isomorphous substitution of metals in hierarchical zeolite
frameworks, their catalytic applications, especially in biomass
conversion, are still in the early stage of development.
(111), (501), and (303), respectively, without the interference of
[18]
other crystalline phases (Figures 1 and S1). Obviously, there is
no significant change of the relative crystallinity when increas-
ing the metal content in the range of 0.1 to 0.7 wt.%. These
observations demonstrate that the crystalline structure can be
preserved in the presence of metals, which might be located
inside the zeolite framework. In addition, to confirm the
incorporation of metals into the zeolite framework, the
magnified XRD patterns in the 2θ range of 7.0 to 10.0°
In this contribution, we report the fabrication and structural
analysis of the isomorphous substitution of various metals
including germanium (Ge), zirconium (Zr), and tin (Sn) into
zeolite frameworks with hierarchical structures. In addition, the
analysis of active sites has been achieved by various numerous
techniques, such as X-ray powder diffraction (XRD), scanning
electron microscopy (SEM), together with energy dispersive X-
ray spectrometry (EDS), transmission electron microscopy (TEM),
UV-Vis spectroscopy, and X-ray photoelectron spectroscopy
demonstrate that almost all the synthesized samples with the
metal incorporation exhibit their XRD characteristic peak (at 2θ
around 8°), shifting approximately 0.05 to 0.1° with respect to
(XPS) to confirm the isomorphous structure of metals species. In
addition, various metals-isomorphous substituted zeolites have
been applied in glucose isomerization to examine the effect of
isomorphous substitution on the catalytic activity.
the non-metal loaded silicalite-1 nanosheets (SiNS). Interest-
ingly, this shifting peak is more obvious with increasing of
metal loading. The reason of XRD peak shifting to lower 2θ
relates to the expansion of unit cell due to the metal
[
19]
substitution in the zeolite framework as shown in Figure 1.
This could be explained from Bragg’s Law equation in which d-
spacing and 2θ are inversely proportional to each other. It is
therefore reasonable to confirm the presence of isomorphous
substitution of metals in zeolite frameworks.
Results and Discussion
Synthesis of metal-isomorphously substituted zeolites with
hierarchical structures
To investigate the morphologies of the as-synthesized
samples, SEM and TEM images are demonstrated as shown in
Figures 2, S2, and S3. The SEM images of all metal substituted
To obtain various metal-isomorphously (Ge, Zr, and Sn)
substituted hierarchical MFI nanosheets (NS), Scheme 1 demon-
Scheme 1. Illustration of preparation scheme demonstrating the synthesis of various metal-substituted MFI silicalite-1 nanosheets.
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Figure 1. (A,C,E) XRD patterns and (B,D,F) magnified XRD patterns in the 2θ range of 7.0 to 10.0° of: (a) SiNS, (b) 0.18%GeMFI-NS, (c) 0.33%GeMFI-NS,
(d) 0.53%GeMFI-NS, (e) 0.14%ZrMFI-NS, (f) 0.33% ZrMFI-NS, (g) 0.73%ZrMFI-NS, (h) 0.17%SnMFI-NS, (i) 0.31%SnMFI-NS, and (j) 0.59%SnMFI-NS.
MFI nanosheets illustrate the identical raspberry-like morphol-
ogy with the nanoparticles-stacking structure with the particle
size distribution in the range of 80 to 150 nm (Figure S4). The
particle sizes of GeMFI-NS samples are relatively smaller than
those of SnMFI-NS and ZrMFI-NS samples. For example, the
particle sizes of the 0.53%GeMFI-NS, the 0.73%ZrMFI-NS, and
the 0.59%SnMFI-NS samples are approximately 85ꢀ12, 152ꢀ
hierarchical structures demonstrate the integrated type I and IV
isotherms containing the high adsorption at low relative
pressure and the hysteresis loops at 0.9 to 1.0 of P/P . These
0
results confirm the presence of both microporous structure due
to a typical zeolite feature of MFI and a hierarchical structure of
nanosheets, eventually leading to an increased external surface
area and external pore volume compared with the conventional
silicalite-1. For example, the specific surface area (SBET) and
2
6, and 118ꢀ18 nm, respectively. Moreover, the sheet thick-
ness of all samples is in the range of 5 to 7 nm (Figure S5).
Additionally, to investigate in term of the metal distribution,
SEM-EDS images illustrate high metal dispersion over the entire
area of zeolite surfaces (Figure 2(d–f), and S6). As expected, TEM
images of all samples clearly explain nanosheet morphology
without the aggregation of metal oxides on the external surface
of zeolites. These observations indicate that metal species are
external pore volume (V ) of the 0.53%GeMFI-NS, the 0.73%
ext
ZrMFI-NS, and the 0.59%SnMFI-NS samples are 570, 486, and
2
À 1
3
À 1
562 m g , and 1.16, 1.09, and 0.84 cm g , respectively, while
2
À 1
the 0.53%SnMFI-CON possesses 404 m g
0.13 cm g of V . Interestingly, when the Sn content of
of SBET and
3
À 1
ext
SnMFI-NS is increased from 0.17 to 0.59 wt.%, the SBET area and
2
À 1
micropore volume (Vmicro) are enhanced from 510 to 562 m g
[19b]
3
À 1
highly dispersed in the zeolite framework.
and from 0.11 to 0.14 cm g , respectively. This could be
attributed to the smaller particle size and sheet thickness, which
were obtained when increasing the Sn content as can be seen
in SEM, TEM images, and particle size distribution results, while
To reveal the textural properties of synthesized zeolites, the
nitrogen adsorption-desorption isotherms are shown in Fig-
ure 3, Table 1, and Table S1. All the synthesized samples with
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Figure 2. (a–c) SEM images, (d–f) EDS images of corresponding metals (Ge, Zr, and Sn), and (g–i) TEM images of (a,d,g) 0.53%GeMFI-NS, (b,e,h) 0.73%ZrMFI-
NS, and (c,f,i) 0.59%SnMFI-NS.
Figure 3. (A) Nitrogen adsorption-desorption isotherms and (B) NLDFT pore size distribution over different catalysts: (a) 0.17%SnMFI-NS, (b) 0.31%SnMFI-NS,
(c) 0.59%SnMFI-NS, (d) 0.53%GeMFI-NS, (e) 0.73%ZrMFI-NS, and (f) 0.53%SnMFI-CON.
Table 1. Textural properties of the prepared catalysts measured by N
2
physisorption.
2
À 1
3
À 1
[g]
Catalysts
Surface area [m g
]
Pore volume [cm g
]
V
ext/total
[a]
[b]
micro
[c]
[d]
[e]
[f]
V
ext
S
BET
S
S
ext
V
total
V
micro
0
0
0
0
0
.17%SnMFI-NS
.31%SnMFI-NS
.59%SnMFI-NS
.53%GeMFI-NS
.73%ZrMFI-NS
510
534
562
570
486
276
295
318
323
298
235
240
244
247
189
1.09
0.97
0.98
1.29
1.21
0.11
0.13
0.14
0.14
0.12
0.97
0.84
0.84
1.16
1.09
0.89
0.86
0.85
0.89
0.90
[
a] SBET: specific surface determined by BET. [b] Smicro: micropore surface area. [c] Sext: external surface area. [d] Vtotal: total pore volume. [e] Vmicro: micropore
volume. [f] Vext: external pore volume (Vext =VtotalÀ Vmicro). [g] Fraction of mesopore and macropore volume (external volume).
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their relative crystallinity was decreased. In addition, the NLDFT
and BJH pore size distributions (PSD) of the metal isomorphous
substituted MFI nanosheets demonstrate that they contain both
micropores and mesopores (Figure 3 and S7). In contrast, the
obviously exhibits the main characteristic peaks at approx-
imately 200 nm, which could refer to the LMCT, ligand-to-metal
4
+ [24]
charge transfer from oxygen framework to Zr
,
indicating
the incorporation of Zr into the zeolite framework with a minor
contribution of bulk ZrO2 at approximately 250 to 300 nm,
0
.53%SnMFI-CON contains a single characteristic of a micro-
[
24b,25]
porous structure. From these observations, it can be concluded
that the hierarchical samples greatly enhance external surface
area due to the interparticle voids of nanolayers.
corresponding to octahedral coordination of ZrO species.
2
It should be noted that the Zr loading on
a zeolite
demonstrates the main characteristic of Zr in tetrahedral form.
These observations confirm the Zr isomorphous substitution in
the zeolite framework for the prepared samples obtained via a
one-pot synthesis method. Moreover, the SnMFI-NS catalysts
also exhibit characteristic peaks at approximately 220 nm,
[15a,20]
To study the acid properties,
the NH -TPD profiles are
3
displayed in Figure S8 and Table S2. All catalysts exhibit two
distinct peaks, which can be assigned to weak acid sites
appeared at the temperature of 100 to 300°C, and the peak at
the higher temperature ranging from 300 to 700°C, containing
2
À
4+
which is attributed to the O
to Sn
charge transfer of
various species of medium and strong acid sites. The results
illustrate that the 0.18%GeMFI-NS, the 0.33%GeMFI-NS, and the
isolated tetrahedral coordination in the framework and charac-
teristic peaks in the range of 250 to 400 nm, which could be
0
.53%GeMFI-NS contain the similar total acid amount, which is
referred to charge transfer of the SnÀ OÀ Sn located outside the
À 1
[26]
in the range of 135–148 μmolg . For ZrMFI-NS catalysts, the
framework structure or SnO .
To further investigate, the
2
0
.14%ZrMFI-NS, the 0.34%ZrMFI-NS, and the 0.73%ZrMFI-NS
0.17%SnMFI-NS, which contains the lowest Sn loading content
exhibits predominantly the tetrahedral Sn. However, when
increasing the Sn content, the 0.31%SnMFI-NS and the 0.59%
À 1
illustrate the total acid amount of 141, 171, and 142 μmolg ,
respectively. Intriguingly, the low acid amount of 142 μmolg
À 1
was obtained when the highest percentage of Zr was
introduced to the zeolite (0.73 wt.%). This phenomenon might
be attributed to the presence of higher portion of zirconium
oxide than the tetrahedrally coordinated Zr, which clearly
indicates that the acid density is not only greatly affected by
metal amount but also the metal species and dispersion. In
SnMFI-NS demonstrate the combination of SnO together with
2
tetrahedral species. Moreover, Tauc plots were applied to
demonstrate the shifted extract edge energy of the samples
obtained from a one-pot synthesis and an impregnation
[15a,27]
method (Figure S11 and Table S3).
Compared with the
sample obtained from the impregnation method (0.50%Sn(imp)
MFI), which mainly composes of bulk octahedral tin species, the
higher edge energy is observed (5.62 and 4.45 eV for SnMFI-NS
and 0.50%Sn(imp)MFI, respectively). It can be concluded that
Sn has also been isomorphously substituted into the zeolite
framework by a one-pot hydrothermal synthesis when using
small metal loading.
addition, SnMFI-NS catalysts possess the total acid density of
À 1
1
41, 160, and 181 μmolg for the 0.17%SnMFI-NS, the 0.31%
SnMFI-NS, and the 0.59%SnMFI-NS, respectively, which slightly
increased when increasing the metal content. However, the
À 1
0
.53%SnMFI-CON exhibits only 125 μmolg of the total acid
densities calculated from the characteristic peak at the temper-
ature ranging from 100 to 600°C. This could also be attributed
To further confirm the oxidation states of metal species in
zeolite, XPS spectroscopy measurement was performed as
shown in Figure 4. For the 0.53%GeMFI-NS, as shown in
Figure 4(A), the Ge species is overlapped with O2 s. However,
for the deconvoluted peaks, it was found that there are two Ge
species appeared at the binding energies of 32.6 eV and
29.3 eV, attributing to the Ge species and Ge , respectively.
For the 0.73%ZrMFI-NS sample, it contains two different species
of Zr as can be seen by the unique characteristic peaks at the
to the low dispersion of metal on the conventional support,
which would affect its acidity.
[21]
Moreover, FTIR spectroscopy of pyridine adsorption was
applied to characterize Lewis acid property of the metal
isomorphously substituted MFI nanosheets as shown in Fig-
ure S9. It clearly shows that the synthesized samples (0.59%
SnMFI-NS, 0.53%GeMFI-NS, 0.73%ZrMFI-NS, and 0.53%SnMFI-
4
+
0
[28]
À 1
CON) possess the characteristic band at 1447 cm , belonging
to Lewis acid sites but there is no characteristic peak at
binding energies of 183.1 eV and 185.3 eV, attributing to 3d5/2,
À 1
[22]
1
545 cm of Bronsted acid sites.
Furthermore, there are
and 3d3/2, respectively, contributing to the tetrahedrally coordi-
nated Zr in zeolite framework and other peaks appear at the
lower binding energies of 182.5 eV and 184.8 eV, which could
À 1
some characteristic peaks at 1597 and 1613 cm , which might
be related to H-bonded pyridine on surface silanol groups.
[23]
[29]
These observations indicate that the synthesized samples
mainly contain the Lewis acid sites, which are typically
considered as active species for glucose isomerization.
be ZrÀ O species of bulk ZrO . The higher binding energy of
2
tetrahedral coordination of Zr with respect to bulk ZrO could
2
be described due to higher positive charge of Zr in the
[
30]
As stated above, the different acid densities of the
synthesized metal-isomorphous substituted zeolites would
derive from different metal species and dispersion. In order to
further confirm the isomorphous substitution of metals in the
zeolite framework, all of the prepared samples were also
characterized by UV-Vis spectroscopy to observe the metal
species (Figure S10). Prior to the measurement, catalysts were
pretreated at 300°C for 12 h under vacuum to remove moisture
framework compared with the Zr located in extra framework.
For SnMFI-NS catalysts, the 0.17%SnMFI-NS, the 0.31%SnMFI-
NS, and the 0.59%SnMFI-NS exhibit three main characteristic
peaks at the binding energies approximately 484.5, 486.5, and
487.4 eV (3d5/2), which can be described as metallic Sn, SnO ,
2
19b]
[
and tetrahedrally coordinated Sn species, respectively.
However, the main characteristics are in tetrahedrally coordi-
nated Sn species as shown in Table S4 demonstrating the ratio
of tetrahedrally coordinated Sn species (487.4 eV) to SnO2
and impurity. It was found that the 0.73%ZrMFI-NS catalyst
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Figure 4. XPS spectra of (A) 0.53%GeMFI-NS (B) 0.73%ZrMFI-NS (C) 0.17%SnMFI-NS, (D) 0.31%SnMFI-NS, and (E) 0.59%SnMFI-NS.
species (486.5 eV) above 1. In addition, this ratio is slightly
decreased with an increase in Sn content. Moreover, the 0.53%
SnMFI-CON exhibits the lower ratio of tetrahedrally coordinated
catalyst (Figure 5 and Table S5). For GeMFI-NS samples, they
exhibit quite low performance in glucose conversion, and it is
similar to what has been observed in the case of bare SiNS
catalyst. For example, with increasing the Ge loading content to
0.53 wt.%, the glucose conversion and the fructose selectivity
are built up to only 18.6% and 21.7%, respectively (Figure 5).
This could be attributed to the equivalent acid amount of all
Sn to SnO species as shown in Figure S12 and Table S4 with
2
respect to SnMFI-NS with the similar metal content. Therefore,
these observations confirm that all the prepared samples
contain both tetrahedrally and octahedrally coordinated Sn
species. Indeed, both types of Sn sites, including tetrahedral
À 1
the GeMFI-NS samples (135–148 μmolg ) compared with the
À 1 [33]
and extra-framework SnO species can convert glucose to
SiNS (70 μmolg ) as shown in Table S2. In addition, Lewis
2
31]
[
[8b]
fructose in aqueous media.
acid strength of Ge has been considered as quite similar as Si.
For the ZrMFI-NS cases, the 0.14%ZrMFI-NS exhibits moderate
glucose conversion and fructose selectivity of 26.7% and
35.8%, respectively. This might relate to the high portion of
tetrahedrally coordinated Zr species. However, when increasing
the amount of Zr to 0.73 wt.%, the relatively low catalytic
activity could be observed with the glucose conversion and
fructose selectivity of 6.4% and 36.3%, respectively, because it
composes of higher fraction of ZnO2 species (Figure 4). For
SnMFI-NS catalysts, the sample with a low amount of Sn catalyst
(0.17%SnMFI-NS) performs a very low catalytic activity of 3.7%
of glucose conversion with 63.6% of fructose selectivity. These
observations relate to low Lewis acid amount of the tetrahe-
drally coordinated Sn (Figure S9 and Table S2). By increasing
the Sn content, SnMFI-NS catalysts exhibit a higher catalytic
activity. Interestingly, the 0.59%SnMFI-NS performs the highest
glucose conversion and fructose selectivity of 31.4% and 87.1%
with the yield almost 30%, respectively, attributing to the
highest number of strong acid sites (Figure S9 and Table S2),
which are the most selective species for glucose
Catalytic testing for glucose isomerization to fructose
To demonstrate the beneficial effect of the metal isomor-
phously substituted zeolites with hierarchical structures, the
glucose isomerization to fructose using water (5 wt.%)/dioxane
system as a solvent was chosen as a model reaction. Indeed, it
has been known that dioxane is a cost-effective solvent for
continuous HMF production from fructose, which is the major
product from glucose isomerization. Thus, using dioxane as a
solvent is an alternative choice, which could facilitate the next
step of reaction for HMF production. In addition, by using the
[30]
mixed solvent with water, it can enhance the fructose yield.
Generally, glucose isomerization to fructose using Lewis acid
catalysts can be occurred via 1,2 intramolecular hydride transfer
following the previous works on both tetrahedral and extra-
[31,32]
framework SnO species.
In this work, the metals isomor-
2
phously substituted MFI catalysts were tested in glucose
[33]
isomerization at 90°C for 12 h using 1 wt.% of glucose in 15 ml
isomerization.
Moreover, in this case the catalytic activity
of water (5 wt.%)/dioxane. For the control experiment, it was
found that the reaction cannot proceed in the absence of a
advances toward the equilibrium conversion of glucose, which
[11a,34]
is 56% with high fructose selectivity.
Compared to a
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Figure 5. (A) Fructose selectivity (red bar), glucose conversion (black bar) and equilibrium conversion (green line), and (B) fructose yield (blue bar) over
different catalysts: (a) blank, (b) SiNS, (c) 0.18%GeMFI-NS, (d) 0.33%GeMFI-NS, (e) 0.53%GeMFI-NS, (f) 0.14%ZrMFI-NS, (g) 0.33% ZrMFI-NS, (h) 0.73%ZrMFI-NS,
(
i) 0.17%SnMFI-NS, (j) 0.31%SnMFI-NS, (k) 0.59%SnMFI-NS, and (l) 0.53%SnMFI-CON. (Reaction condition: a 0.3 g of catalyst, 1 wt.% glucose in water (5 wt.%)/
dioxane at 90°C for 12 h of reaction time)
conventional zeolite, the 0.53%SnMFI-CON exhibits a very low
glucose conversion at 3.8% with fructose selectivity of 86.8%,
which could be attributed to the low amount of strong acid
site, resulting in low catalytic performance in glucose isomer-
ization. Moreover, the conventional silicalite-1 containing a sole
microporous structure with 0.55 nm of microporous channel
framework together with the nanosheet structure of zeolite
catalysts utilizing glucose isomerization. It opens up a potential
route to further study metal substituted zeolites for many
applications in the future.
could not facilitate the glucose transportation to the active Experimental Section
sites, resulting in the low catalytic activity. Indeed, the kinetic
diameter of glucose is approximately 0.85 nm, which is larger
Catalyst preparation
[35]
than zeolite micropores.
However, the catalytic activity is
Silicalite-1 nanosheets (SiNS) and metals substituted MFI nano-
sheets were manufactured via a one-pot hydrothermal method.
significantly improved when applying the SnMFI-NS containing
hierarchical structures. These observations confirm that the
reaction is probably proceeded on external surface of the
zeolite especially between nanosheet layers.
[2a]
Typically, a 4.34 g of TEOS (tetraethyl orthosilicate) as a silica source
taken from Sigma-Aldrich was mixed with a metal source including
germanium oxide (Alfa Aesar, 99.999% metals basis), zirconyl (IV)
nitrate solution (ACROS Organics, 99.5%) or tin (IV) chloride
pentahydrate (Sigma-Aldrich, 98%) solution with varying the
loading content of MO /SiO molar ratio ranging from 0 to 0.04.
x
2
Conclusion
Subsequently, the prepared precursor was stirred with adding a
.06 g of 40 wt.% of tetra(n-butyl)ammonium hydroxide in H O
4
2
In this work, various metal-isomorphous substituted MFI nano-
sheets were successfully synthesized, including GeMFI-NS,
ZrMFI-NS, and SnMFI-NS using a one-pot hydrothermal method.
Based on various characterization results, it was confirmed that
the isomorphous substitution into the zeolite framework has
been achieved. XRD spectra clearly illustrate the characteristic
peak shifting to lower 2θ degrees approximately 0.05 to 0.1°,
(TBAOH, Leonid) as a structure-directing agent (SDA). After that, a
sodium hydroxide solution, a 0.01 g of NaOH (Carlo Erba) in H O,
2
was added under vigorous stirring, then stirred continuously for
1
1
1
2 h to the above-mentioned mixture. The total amount of H O is
2
.32 g. The obtained precursor was then kept in the autoclave at
30 C for 2 days. After that, the synthesized compound was
°
washed with DI water to get rid of the excess basic fragments,
dried at 100°C, and calcined to remove the template at 550°C for
6
h. The prepared zeolites are named as x% GeMFI-NS, x%ZrMFI-NS,
confirming the larger unit cell of metal-isomorphous framework
with respect to the unmodified zeolite. SEM and TEM images as
well as SEM-EDS mapping analysis reveal the nanosheet
structure with high dispersion of metals. Moreover, UV-Vis and
XPS spectra can confirm the presence of tetrahedral framework
of metals substituted in a zeolite structure as the main
characteristic. Intriguingly, these isomorphous substituted cata-
lysts have been tested in glucose isomerization, in which the
and x%SnMFI-NS for Ge, Zr, and Sn incorporated in MFI framework,
respectively, where x refers to metal content in wt.%.
In addition, to compare with the bulk zeolite, the isomorphous
substitution of Sn in a conventional silicalite-1 was obtained by a
hydrothermal method. Firstly, the mixture composed of tetraethyl
orthosilicate (TEOS, 3.50 g), a 1 M tetrapropylammonium hydroxide
in H O (TPAOH, Sigma-Aldrich, 1.72 g), tin chloride, and DI water
2
(10 g) was stirred for 10 min. The sodium hydroxide solution (a
0
2
.07 g of NaOH in a 2.27 g of DI water) was mixed and stirred for
h with the previous mixture. Subsequently, it was transferred to
0
.59%SnMFI-NS performs the highest glucose conversion and
fructose selectivity of 31.4% and 87.1%, respectively, attributing
to the highest amount of strong acid sites of Sn tetrahedral
coordination. This work reveals the benefit of the metal
isomorphous substitution of Sn, Ge, and Zr into the MFI zeolite
the autoclave and heated at 180°C for 3 days. After that, it was
filtered until pH less than 7, dried overnight at 100°C and calcined
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to remove the template at 550°C for 6 h. The prepared sample is Acknowledgements
named as the 0.53%SnMFI-CON.
This work was financially supported by the Vidyasirimedhi Institute
of Science and Technology (VISTEC), and the Nanocatalysts and
Nanomaterials for Sustainable Energy and Environment Research
Network (RNN) NANOTEC. C. W. thanks the Mid-Career Research
Grant 2020 from National Research Council of Thailand (NRCT5-
RSA63025-03). This research also received financial support from
Thailand Science Research and Innovation (TSRI) (FRB640087).
Characterization
The X-ray powder diffraction (XRD) using D8 ADVANCE model of
Bruker was applied to characterize the crystallinity of all samples
using step sizes of 0.036° from 5 to 55° of 2θ. To characterize the
elemental compositions, they were characterized using a wave-
length dispersive X-Ray fluorescence spectroscopy (WDXRF) of S8
Tiger model, Bruker. Scanning electron microscope and energy
dispersive X-ray spectrometer (SEM-EDS) were performed using
JEOL JSM-7610F and transmission electron microscopy (TEM)
images were obtained by a JEOL-JEM-ARM2000F to confirm the
morphologies, particle size, and metal distribution of the samples.
Nitrogen adsorption-desorption analysis was used to measure the
textural properties of the synthesized samples at À 196°C using a
Conflict of Interest
The authors declare no conflict of interest.
BELSORP-max model. Before the measurement, zeolites were
Keywords: glucose isomerization
· hierarchical zeolites ·
performed under the vacuum-pretreatment at 300°C for 24 h. The
isomorphous substitution · MFI zeolites · zeolite nanosheets
specific surface area (S ) was estimated from Brunauer-Emmett-
BET
Teller (BET) theory. The total pore volume (V ) was acquired at P/
total
P0 of 0.99. The micropore volume (Vmicro) was estimated using the t-
plot method. The pore size distribution was obtained from the
Barrett-Joyner-Halenda (BJH) method. To measure acid properties
and acid amount, ammonia temperature-programmed desorption
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8
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These are not the final page numbers!

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doi.org/10.1002/cplu.202100289
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© 2021 Wiley-VCH GmbH
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These are not the final page numbers!

FULL PAPERS
Sn, Ge and Zr isomorphously substi-
tuted MFI zeolites with hierarchical
structures have been fabricated. The
MFI nanosheets with appropriate
amounts of Sn could significantly
improve the catalytic activity in
glucose conversion to fructose due
to the major contribution of tetrahe-
drally coordinated Sn in the
framework together with hierarchical
structures, eventually facilitating
molecular transportation of bulky
molecules to active sites.
P. Dugkhuntod, N. Maineawklang, C.
Rodaum, P. Pornsetmetakul, K.
Saenluang, Dr. S. Salakhum,
Prof. Dr. C. Wattanakit*
1
– 10
Synthesis and Characterization of
Sn, Ge, and Zr Isomorphous Substi-
tuted MFI Nanosheets for Glucose
Isomerization to Fructose