Promotion effect of Mg on a post-synthesized Sn-Beta zeolite for the conversion of glucose to methyl lactate

Article abstract of DOI:10.1039/c9cy02376c

Mg-Sn-Beta zeolites with different Mg/Sn molar ratios were prepared from the parent deAl-Beta by a coimpregnation method. The samples were characterized by powder X-ray diffraction (XRD), N₂ physisorption, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy of the -OH region and probe molecule (pyridine, CD₃CN and CHCl₃) adsorption. The results indicate that Mg²⁺ and Sn⁴⁺ can react with silanol nests and incorporate into the framework of Beta zeolite. Moreover, it is relatively facile to incorporate Sn⁴⁺ into the framework. The framework metal sites generate Lewis (L) acid sites; however, the L acid strength of the framework Mg²⁺ is weaker than that of framework Sn⁴⁺. A small amount of Mg²⁺ interacts with isolated silanols to form extraframework species. Therefore, the introduction of Mg²⁺ results in a significant reduction in silanol defects and an increase in L acidity/basicity of Mg-Sn-Beta. Mg-Sn-Beta with Mg/Sn molar ratio of 1 has the least silanol defects. In the conversion of glucose to methyl lactate (MLA), Mg-Beta is less active than Sn-Beta due to its weaker L acidity. The TOF value for MLA formation increased in the order of Sn-Beta < 0.25Mg-Sn-Beta ≈ 4Mg-Sn-Beta < 1Mg-Sn-Beta, which is closely related to the amount of silanol defects in the catalysts. A kinetic study indicates that the apparent activation energy of the retro-aldol of fructose to MLA, which is the rate-determining step of glucose conversion to MLA, decreases over Mg-Sn-Beta compared to that over Sn-Beta and thus, the formation of MLA was promoted.

Full text of DOI:10.1039/c9cy02376c

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Catalysis Science & Technology
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DOI: 10.1039/C9CY02376C
ARTICLE
Promotion effect of Mg on post-synthesized Sn-Beta zeolite for
the conversion of glucose to methyl lactate
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
a
a
Xiaomei Yang, Bin Lv, Tianliang Lu, Yunlai Su and Lipeng Zhou*
DOI: 10.1039/x0xx00000x
Mg-Sn-Beta zeolite with different Mg/Sn molar ratio was prepared from the parent deAl-Beta by coimpregnation method.
The samples were characterized by powder X-ray diffraction (XRD), N physisorption, Ultraviolet-visible diffuse reflectance
spectroscopy (UV-vis DRS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy of −OH
region and probe molecule (pyridine, CD CN and CHCl
) adsorption. The results indicate Mg2+ and Sn4+ can react with
2
3
3
silanol nests and be incorporated into the framework of Beta zeolite; moreover Sn4+ is relatively facile to be incorporated
into the framework. The framework metal sites generate Lewis (L) acid sites, however the L acid strength of framework
Mg2+ is weaker than that of framework Sn . Part of Mg interacts with isolated silanols and formed extraframework
species. Therefore, the introduction of Mg2+ results in significant reduction of silanol defects and increase in L
acidity/basicity of Mg-Sn-Beta. Mg-Sn-Beta with Mg/Sn molar ratio of 1 has the least silanol defects. In the conversion of
glucose to methyl lactate (MLA), Mg-Beta is less active than Sn-Beta due to its weaker L acidity. TOF value for MLA
formation increased in the order of Sn-Beta < 0.25Mg-Sn-Beta ≈ 4Mg-Sn-Beta < 1Mg-Sn-Beta, which is closely related to
the amount of silanol defects of the catalysts. Kinetic study indicates that the apparent activation energy of the retro-aldol
of fructose to MLA, the rate-determining step of glucose to MLA, decreases over Mg-Sn-Beta compared to Sn-Beta, and
4+
2+
thus
the
formation
of
MLA
was
promoted.
the selectivity of Sn-Beta for the conversion of hexose to alkyl
lactate is affected significantly by its preparation method. Sn-
Beta synthesized in fluoride media with less silanol defects
exhibit higher selectivity for alkyl lactate than the post-
1
Introduction
Biomass is an abundant and sustainable resource of the world.
Carbohydrate is a main component of plant biomass. Utilizing
1
1
1,12
synthesized counterpart.
Unfortunately, the synthesis of
carbohydrate as raw material to produce chemicals and
biofuels is an important part of sustainable development.
Glucose is the most abundant and cheap hexose, which can be
produced from inedible cellulose. Many high value-added
chemicals such as lactic acid (LA) and its derivatives have been
produced from glucose. LA and its derivatives have wide
applications in the industries of food, cosmetics,
pharmaceutical, plastics and chemicals. Glucose can be
converted to LA in water or alkyl lactate in alcohol by
chemocatalytic method besides microbial fermentation.
Lewis (L) acid and base are two kinds of catalysts for
conversion of glucose to LA or alkyl lactate.
acid is more selective than base for producing alkyl lactate
from hexose in alcohol.
Sn-Beta molecular sieve with strong L acidity and large
three-dimension micropore shows excellent performance for
transforming carbohydrate to LA and alkyl lactate. However,
Sn-Beta in fluoride media is time-consuming and fluoride is
1
3
noxious. Post-synthesis method avoids these drawbacks,
which prepared Sn-Beta via dealumination of Al-Beta to
generate vacant T sites and then incorporation of Sn into the
2
,3
1
4
4
vacant T sites. But, the vacant T sites are usually difficult to
be completely occupied by tin element, which leads to more
silanol defects in post-synthesized Sn-Beta (Sn-Beta-P) and
thus lower selectivity in the conversion of hexose to alkyl
,7 lactate. Vega-Vila et al. adopted reflux of vacuum-dried deAl-
5
,6
5
4
Beta and anhydrous SnCl in moisture-free dichloromethane to
1
5
5
,7-9
prepare Sn-Beta. The method can precisely control the
density of framework Sn sites and residual vacancy defects in
Sn-Beta, and every framework vacancy can be occupied by Sn.
This method needs to proceed under anhydrous condition and
noxious organic reagent is used. Addition of promoter to Sn-
Beta is a method to improve its selectivity. Tolborg et al. added
alkali ions to Sn-Beta-P and improved the yield of methyl
lactate (MLA) from 20% to 72% in the conversion of sucrose in
Generally, L
1
0
a. Green Catalysis Center and College of Chemistry, Zhengzhou University, 100 Kexue
Road, Zhengzhou 450001, China. Tel.: +86 371 67781780; E-mail:
11
methanol. Nevertheless, alkali ions will leach into the
reaction solution, affecting the stability and recycling of the
catalyst. Dong et al. used Zn as a promoter of Sn-Beta-P and
increased the yield of LA from 23% to 48% in the conversion of
zhoulipeng@zzu.edu.cn
School of Chemical Engineering, Zhengzhou University, 100 Kexue Road,
b.
Zhengzhou 450001, China.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
1
6
glucose in water. They thought that the introduction of Zn
enhanced both the amount of L acid and base sites; the base
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Page 2 of 10
ARTICLE
Journal Name
sites inhibited side reactions and increased the selectivity of LA. Powder X-ray diffraction (XRD) patterns were obVtieawinAertdicleoOnnlinae
Recently, our group used metal oxide as promoter to improve PANalytical X’pert PRO instrument with DCOuI: 1K0α.10r3a9d/Cia9tCioYn023(λ76C=
the selectivity of Sn-Beta-P for the conversion of glucose to 0.15418 nm). Tube voltage and tube current were 40 kV and 40 mA,
MLA;17 the yield of MLA was increased from 25% over Sn-Beta- respectively. The crystallinities were calculated according to the
P to 52% over WO
WO is large, which is harmful to health. Moreover, the were determined on a Quantachrome Autosorb at -196 °C. Samples
problem of WO leaching at the reaction conditions cannot be were degassed for 3 h at 300 °C before N adsorption. Total surface
3 2
promoted Sn-Beta-P. But, the usage of area of the peaks at 2θ of 7.9° and 22.6°. The physisorption of N
3
3
2
ignored. Therefore, it is necessary to explore a harmless and area was calculated with Brunauer-Emmet-Taller (BET) method.
stable promoter and reveal the effect of the promoter on the Micropore and mesopore distributions were calculated by the
properties of Sn-Beta, which is important for establishing the Horvath−Kawazoe (HK) method and the Barret−Joyner−Halenda
structure-property-performance relationship.
(BJH) method, respectively. Ultraviolet-visible diffuse reflectance
The acidity/basicity of Mg compound is unique, which spectra (UV-vis DRS) were recorded with an Agilent Technologies
2
+
depends on the state of Mg. MgO is a basic oxide, while Mg
Cary Series UV-vis-NIR spectrophotometer and BaSO
Additionally, reference. X-ray photoelectron spectroscopy (XPS) was measured
Mg in the framework site of silicate molecular sieve can on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg
4
was used as a
1
8,19
in the framework MOF can provide L acidity.
2
+
2
0
induce strong basicity of framework oxygen atoms. Thus, Mg Kα radiation (hν = 1253.6 eV) operating at 250 W and 14.0 kV. The
based catalysts are widely used in the conversion of binding energy of C 1s was used as a reference at 284.6 eV. Fourier
carbohydrate.2
1,22
In the present study, Mg was used as a transform infrared (FT-IR) spectra of the −OH region and deuterated
CN), pyridine or CHCl adsorption were recorded on
promoter to improve the catalytic performance of Sn-Beta-P in acetonitrile (CD
3
3
the conversion of glucose to MLA. The state of Mg and Sn and a Bruker Tenser II FT-IR spectrometer. The sample was pressed to a
the acidity/basicity of Mg-Sn-Beta were characterized by self-supporting wafer (~13 mg) and put into an infrared quartz cell.
various techniques. The catalytic performance of Mg-Sn-Beta The wafers were pretreated at 450 °C for 3 h under vacuum. After
was investigated in the conversion of glucose to MLA. The the cell was cooled down to 25 °C, the spectrum of the hydroxyl
2
+
promotion effect of Mg was revealed.
region was recorded, which is also the background for CD
pyridine or CHCl adsorption. Subsequently, CD CN, pyridine or
CHCl vapor was introduced to the quartz cell until the adsorption
reached saturation. The spectra of CD CN or CHCl adsorption were
3
CN,
3
3
2
Experimental section
3
3
3
2
.1 Materials
obtained under vacuum at 25 °C. For the pyridine FT-IR, the sample
was heated to the desired temperature and kept for 0.5 h under
vacuum. The spectrum was recorded after the sample was cooled
down to room temperature.
The parent Al-Beta zeolite with Si/Al molar ratio of 13.8 was
purchased from Nankai University Catalyst Co. (China). 1,3-
Dihydroxyacetone dimer (97%, DHA) was obtained from J & K
Scientific Ltd, China. Methyl lactate (MLA) was purchased from TCI 2.4 Catalytic tests
Shanghai, China. Glucose monohydrate and SnCl ·5H O were
4
2
The conversion of carbohydrate was carried out in a stainless
autoclave (80 mL) with PTFE liner. Typically, the liner was first
loaded with methanol (15 mL), and then carbohydrate (0.37 g) and
catalyst (0.20 g) was added. After the autoclave was sealed, the
purchased from Tianjin Kermel Chemical Reagent, China. Fructose
and mannose were purchased from Aladdin Reagent Co., China.
Nitric acid (65-68%), Mg(NO ) ·6H O, methanol and naphthalene
3 2 2
were purchased from commercial source and were analytical
reagents. All chemicals were used as received without further
purification.
atmosphere over the solution was flushed four times with N
then the pressure of N was charged to 0.4 MPa. Subsequently, the
reactor was heated to the desired temperature at a heating rate of
2
, and
2
−1
2
.2 Preparation of Mg-Sn-Beta
5 °C min and kept the temperature for a designed time. When the
reaction was finished, the reactor was cooled down to the ambient
temperature in ice water bath. The reaction mixture was
centrifuged and the obtained liquid was collected and adjusted to
constant volume with methanol for GC and HPLC analyses. GC
analysis was carried out on a Shimadzu GC-2014C equipped with an
FID detector and naphthalene was used as the internal standard. LC
analysis was performed on a Shimadzu LC-20AT HPLC analysis
system equipped with an Aminex HPX-87H column (300 mm × 7.8
The parent Al-Beta zeolite was dealuminated with nitric acid (13
mol·L , 20 mL·g zeolite) at 100 C for 20 h. After filtration and
washing by deionized water until the filtration was near neutral, the
dealuminated Beta (deAl-Beta) was dried at 100 C overnight. Mg-
Sn-Beta was prepared by incipient wetness impregnation of
–1
–1
o
o
SnCl
4
·5H
2
O and Mg(NO
3
)
2
·6H
2
O aqueous solutions. The mixture was
o
aged at room temperature overnight, dried at 100 C and then
o
calcined at 550 C for 5 h. These samples were denoted as xMg-Sn-
Beta, where x is the molar ratio of Mg/Sn. The value of x is 0.25, 0.5,
, 2 and 4, respectively. The nominal Sn content in xMg-Sn-Beta is 2
wt%. As comparison, Sn-Beta and Mg-Beta were also prepared by
the same method; the content of Sn or Mg was identical with that
in 1Mg-Sn-Beta.
−1
mm) and refractive index detector (RID-10A). H
2 4
SO (5 mmol L )
was used as the mobile phase under the column temperature of
1
−1
8
0 °C, which had a flow rate of 0.6 mL min . Carbohydrate
conversion and product yield were calculated as follows. All product
yields are carbon yields.
2
.3 Characterization
2
| J. Name., 2012, 00, 1-3
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ARTICLE
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DeAl-Beta
Mg-Beta
DOI: 10.1039/C9CY02376C
(a)
4
Mg-Sn-Beta
Mg-Sn-Beta
Sn-Beta
0.25Mg-Sn-Beta
-1
1
00 mL g
1
4
Mg-Sn-Beta
Mg-Sn-Beta
1
0
.25Mg-Sn-Beta
Sn-Beta
Mg-Beta
deAl-Beta
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
10
20
30
40
50
2Theta degree
0.5
(
b)
deAl-Beta
Mg-Beta
Sn-Beta
Fig. 1 XRD patterns of deAl-Beta, Mg-Beta, Sn-Beta and Mg-Sn-Beta
zeolites.
0
0
0
0
0
.4
0.25Mg-Sn-Beta
1
4
Mg-Sn-Beta
Mg-Sn-Beta
.3
.2
.1
.0
0
10
20
30
40
50
60
C C
where N and N ′ is the number of carbon atoms in a carbohydrate
Pore diameter (nm)
molecule and a product molecule.
0
0
0
0
.15
.10
.05
.00
(
c)
3
Results and discussion
deAl-Beta
Mg-Beta
Sn-Beta
0
1
4
.25Mg-Sn-Beta
Mg-Sn-Beta
Mg-Sn-Beta
3
3
.1 Characterization of Mg-Sn-Beta zeolite
.1.1 Crystalline phase and texture properties
XRD patterns of the samples are shown in Fig. 1. All of the samples
exhibit well-defined reflections with a *BEA topology. No diffraction
peaks assigned to SnO and MgO are observed, indicating
2
0
.4
0.6
0.8
1.0
1.2
1.4
1.6
Pore diameter (nm)
2
Fig. 2 N isotherms (a), BJH (b) and HK (c) derived pore size
distributions of the samples.
Table 1 Physicochemical properties of deAl-Beta, Mg-Beta, Sn-Beta
and Mg-Sn-Beta zeolites
Sample
S
BET
2
S
(m
g )
ex
V
tot
V
micro
V
meso
Crys L acid
talli densit
2
–
–
(m
(mL
(mL g
(mL g
Sn and Mg species are highly dispersed. The relative crystallinity of
the samples is listed in Table 1. The crystallinity of Sn-Beta
decreases remarkably compared to deAl-Beta probably due to the
–1
–1 a
-1
1
1 a
g )
g )
)
)
nity
y
b
(%)
(umol
–
1 c
g )
corrosion of acidic SnCl
4
aqueous solution. Contrarily, the
Al-Beta
deAl-
Beta
Mg-
Beta
511
534
154
160
0.55 0.18
0.58 0.19
0.37
0.39
156
100
–
0
crystallinity of Mg-Beta increases compared to deAl-Beta, indicating
the structure of deAl-Beta is reconstructed during preparation in
2+
the presence of Mg . Alkali and/or alkaline earth metal cations as
513
147
0.51 0.19
0.32
122 34
inorganic structure-directing agents (ISDAs) have been widely
23
2+
studied. Here, Mg probably plays a structure-directing role. The
crystallinity of Mg-Sn-Beta increases gradually with the amount of
Mg in comparison with Sn-Beta due to the structure-directing role
Sn-Beta 524
.25Mg 515
Sn-
Beta
Mg-
Sn-Beta
Mg-
177
157
0.55 0.18
0.51 0.20
0.37
0.31
46
45
55
85
0
-
2+
of Mg .
The results of N
2
physisorption (Table 1 and Fig. 2) show that
1
515
480
164
157
0.54 0.18
0.52 0.17
0.36
0.35
63
68
83
these samples are microporous materials with a certain amount of
mesopores (~9.6 nm) generating from dealumination. The
micropore size accords with that of BEA* zeolite. The BET surface
area, total pore volume and mesopore volume of Mg-Beta and Sn-
Beta decrease slightly compared with deAl-Beta. With the increase
of Mg content, these parameters of Mg-Sn-Beta also decrease
slightly compared to Sn-Beta except for 4Mg-Sn-Beta. For 4Mg-Sn-
Beta, the BET surface area reduces significantly due to the high
content of Mg, implying that there are some Mg species such as
small MgO clusters in pores leading to pore blocking to some extent.
4
111
24
Sn-Beta
a
External surface area = BET surface area - micropore surface area;
mesopore volume = total pore volume - micropore volume, where
the micropore surface area and volume were determined by the t-
plot method at a relative pressure of 0.05-0.70. b Calculated
c
according to the area of the peaks at 2θ of 7.9° and 22.6°.
3
1
Calculating according to the equation given in the literature.
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C9CY02376C
3
692
deAl-Beta
Mg-Beta
Sn-Beta
Mg-Beta
Sn-Beta
(A)
0
1
4
.25Mg-Sn-Beta
Mg-Sn-Beta
Mg-Sn-Beta
3
500
0
1
4
.25Mg-Sn-Beta
Mg-Sn-Beta
Mg-Sn-Beta
2
00
250
300
350
400
Wavelength (nm)
4
000
3800
3600
3400
3200
)
3000
-1
Wavenumber (cm
2
00
300
400
500
600
700
800
Wavelength (nm)
3
692
(
B)
Fig. 3 UV-vis DR spectra of zeolites.
a
b
3
500
3
.1.2 State of Mg and Sn in Beta zeolite
UV-vis DR spectra of Mg-Beta, Sn-Beta and Mg-Sn-Beta are given in
Fig. 3. All the zeolites show an absorption band at 200 nm, which is
different from bulk SnO and MgO. SnO exhibits a wide absorption
2 2
band at the range of 200-300 nm (Fig. S1a). MgO presents two weak
absorption bands at 210 nm and 279 nm (Fig. S1b). This indicates
the state of Mg and Sn species in zeolite is different from that in
bulk oxide. Probably, Mg and Sn species locate in the framework
position of Beta zeolite, but the presence of extraframework Mg
and Sn species are not excluded.
4
000
3800
3600
3400
3200
3000
-1
Wavenumber (cm )
Fig. 5 FT-IR spectra (A) in the hydroxyl stretching vibration region of
zeolites and the difference spectra (B) of deAl-Beta with Mg-Beta (a)
and Sn-Beta (b).
XPS was also conducted to examine the chemical states of Sn and
Mg (Fig. 4). In the Sn 3d region (Fig. 4a), two signals positioned at
4
2
86.6 and 495.0 eV are observed for SnO , which are characteristic
of Sn sites of crystalline tin dioxide.25-27 For Sn-Beta, two bands at
higher binding energy (487.7 and 496.2 eV) are found, which are
ascribed to 3d5/2 and 3d3/2 photoelectrons of tetrahedrally
2
5-27
coordinated framework Sn species.
photoelectrons in 1Mg-Sn-Beta is 0.3 eV lower than that of Sn-Beta,
but it is still much higher than that of SnO . This suggests that Sn is
The binding energy of Sn 3d
4
87.4
(a)
4
95.9
2
1
Mg-Sn-Beta
Sn-Beta
incorporated into the framework sites of 1Mg-Sn-Beta. In the Mg 2p
region (Fig. 4b), MgO exhibits a narrow peak at 49.3 eV, which is
consistent with that reported in literature. In contrast, Mg 2p
peaks for Mg-Beta and 1Mg-Sn-Beta are wide and center at higher
binding energy. This suggests the chemical state of Mg species in
the two samples is nonuniform.
487.7
496.2
2
8
4
86.6
4
95.0
SnO2
480
3
.1.3 Acidity and basicity of Mg and/or Sn-Beta zeolites
5
00
495
490
485
475
Binding energy (eV)
Fig.5A shows the FT-IR spectra in the hydroxyl stretching vibration
−1
region. The band at the range of 3600-3800 cm corresponds to
−1
isolated silanols and the band at 3400-3600 cm is assigned to
(
b)
50.8
2
9
silanol nests. Compared to deAl-Beta, the introduction of Mg
and/or Sn reduces both the isolated silanols and the silanol nests. It
is considered that metal reacting with silanol nests forms
framework metal sites and metal reacting with isolated silanols
generates extraframework species. Therefore, both Mg and Sn can
1
Mg-Sn-Beta
50.0
49.3
Mg-Beta
2
+
be incorporated into the framework of Beta zeolite. Mg species
interacts with silanol nests in two possible forms as shown in
MgO
2
+
Scheme 1. Mg fully incorporated into the framework (Species I)
2
+
resulting in two negative charges that balanced by one Mg cation.
6
5
60
55
50
45
40
2
+
The other is Mg condensed with two hydroxyl groups of silanol
nests (Species II). Here, we called this form as quasi-framework Mg
Binding energy (eV)
Fig. 4 XPS spectra of Sn 3d (a) in SnO
and Mg 2p (b) in MgO, Mg-Beta and 1Mg-Sn-Beta.
2
, Sn-Beta and 1Mg-Sn-Beta
2
+
species. Zn incorporated into Beta framework was also reported
4
| J. Name., 2012, 00, 1-3
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Mg2+
H
View Article Online
DOI: 10.1039/C9CY02376C
Si O-
Si O
Si
(
I)
O-
(II)
O Si
Si
OH
1598
1612
1
449 1446
Mg
1492 1452
deAl-Beta
Mg-Beta
Mg
O Si
1610
Si O
O Si
Framework Mg with Mg2+
to balance negative charge
Sn-Beta
Quasi-framework Mg species
0
.25Mg-Sn-Beta
1
Mg-Sn-Beta
(
IV)
(
III)
Mg
4
Mg-Sn-Beta
1550
(
MgO)n
Si O
O Si
1650
1600
1500
1450
1400
Small MgO cluster
-1
)
Wavenumber (cm
Ion-exchange-like Mg species
Scheme 1 Possible Mg2+ species in Mg-Sn-Beta zeolite.
Fig. 7 FT-IR spectra of pyridine adsorbed on different Beta zeolites
after evacuation at 150 C for 30 min.
o
in similar forms.30 Mg2+ interacting with two isolated silanols in
vicinity maybe form Species III, a type of extraframework Mg
2
+
peak at 2305 cm−1 is corresponding to framework Mg2+ sites. The
species. The difference spectra of deAl-Beta with Mg-Beta or Sn-
Beta are shown in Fig. 5B. It can be seen that Sn incorporation
consumes more silanol nests, whereas Mg introduction consumes
more isolated silanols. This implies that Sn is relatively facile to be
incorporated into the framework sites. Therefore, the amount of
2
+
lower wavenumber suggests the L acidity of framework Mg sites is
2
+
weaker than framework Sn sites. Mg centers as L acid sites are
1
8,19
also reported in Mg-MOF-74.
The intensity of the peak at 2305
−1
2+
cm increases with the increase of Mg content in Mg-Sn-Beta,
meaning that more Mg is incorporated to the framework sites and
forms more L acid sites.
2
+
2
+
the extraframework Mg species cannot be ignored. For Mg-Sn-
Beta, the amount of silanols decreases first and then increases with
the amount of Mg. 1Mg-Sn-Beta has the least silanols. The more
silanol defects in 4Mg-Sn-Beta than 1Mg-Sn-Beta is due to that
more small MgO clusters formed at high content of Mg as proved
The acidity of Mg-Sn-Beta was also monitored with FT-IR spectra
of pyridine adsorption (Fig. 7). DeAl-Beta shows two the peaks at
−
1
27
1
446 and 1598 cm corresponding to hydrogen-bonded pyridine.
−1
2
+
For Sn-Beta, other two peaks at 1452 and 1612 cm are also
observed, which are ascribed to the vibration of pyridine on L acid
by the results of N
2
physisorption and thus less Mg interacts with
silanols.
2
8-30,32,33
sites.
610 cm , respectively. This proves again that the L acidity of Mg-
Beta is weaker than Sn-Beta. On the basis of the area of the peak at
For Mg-Beta, the two bands red-shift to 1449 and
All coordinatively unsaturated metal sites can serve as L acid sites.
CD CN is considered to be an efficient and sensitive probe molecule
to framework metal sites and their L acidity.
of CD CN adsorption of the samples is given in Fig. 6. The
absorption peak at 2274 cm is corresponding to CD
−1
1
3
3
1,32
FT-IR spectroscopy
−1
1
452 cm , the density of L acid sites was calculated and listed in
Table 1. It can be seen that the L acid density increases with the
increase of Mg content. This is consistent with the results of CD CN
3
−
1
3
CN adsorbed
Besides this peak, Sn-Beta shows another
3
2
7-29,31,32
onto silanols.
absorption peak at 2310 cm , which is assigned to CD
interacting with framework Sn sites.
adsorption. The amount of Brønsted acid sites of all the samples
−1
−1
3
CN
Mg can form hexa-
(
~1550 cm ) can be negligible.
4
+
27-29,31,32
2+
The basic properties of the zeolites were examined by FT-IR
2
+
coordinated compound such as MgO, so framework Mg can also spectroscopy of CHCl
3
adsorption (Fig. 8). Peak position of the C-H
accept electron pair and act as Lewis acid sites. For Mg-Beta, the
2
925
2
957
3
026
022
deAl-Beta
Mg-Beta
3
2305
2310
deAl-Beta
Mg-Beta
0.05
Sn-Beta
0.25Mg-Sn-Beta
2274
Sn-Beta
1
4
Mg-Sn-Beta
Mg-Sn-Beta
0
.25Mg-Sn-Beta
1
Mg-Sn-Beta
4
Mg-Sn-Beta
3100
3050
3000
2950
2900
2850
2800
-1
Wavenumber (cm )
2
340 2320 2300 2280 2260 2240 2220 2200
-1
Wavenumber (cm )
3
Fig. 8 FT-IR spectra of CHCl absorbed on deAl-Beta, Mg-Beta, Sn-
3
Fig. 6 FT-IR spectra of CD CN absorbed on deAl-Beta, Mg-Beta, Sn-
Beta and Mg-Sn-Beta. Spectra were obtained at room temperature
after desorption for 15 min under vacuum.
Beta and Mg-Sn-Beta. Spectra were obtained at room temperature
after desorption for 20 min under vacuum.
−1 33,34
stretching in the liquid CHCl
3
is at 3033 cm .
This band shifts to
−1
lower wavenumbers (3026 cm ) after CHCl
3
adsorbed on the 4Mg-
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Sn-Beta with the highest Mg content, this band shifts to 3022cm , which is consistent with that reported in literature.11,12,17 The
−1
View Article Online
moreover, the intensity of this band is the highest among these conversion of glucose and the yield of MLA DoOveI:r1M0.1g0-3B9e/tCa9aCrYe02lo3w76eCr
samples, indicating that 4Mg-Sn-Beta has the strongest basicity. The than those over Sn-Beta, due to the lower L acidity of Mg-Beta. But
−1
band at 2957 cm can be attributed to CHCl
3
adsorbed with the Mg-Beta is more selective for the formation of MLA than MgO. Mg-
mode of species I. The weaker the C-H bond, the lower Beta mainly exhibits L acidity, while MgO is a typical solid base.
3
3
wavenumber of the peak of C-H stretching. So, the band at 2925 Both the activation mode for substrate and the formation
−
1
8,35
cm probably caused by CHCl
3
adsorbed via the mode of species III mechanism of MLA over L acid and base are different.
L acid
generally has higher selectivity than base for the conversion of
decreases while the intensity of the bands at 2957 cm and 2925 carbohydrate to alkyl lactate. Our recent work indicates that
−1
in Fig. S2. For all the samples, the intensity of the band at 3026 cm
−1
−1
3
cm increase with the lapse of desorption time (Fig. S3). This addition of some oxide such as WO as a co-catalyst can improve
1
7
indicates that the interaction of CHCl
modes of species I and III is strong.
3
with zeolite through the remarkably the selectivity of Sn-Beta for MLA, but MgO as a co-
catalyst hardly works. The yield of MLA over MgO combined Sn-
Beta binary catalyst is similar with that over Sn-Beta alone. It has
3
3
.2 Catalytic performance of Mg-Sn-Beta zeolite
.2.1 Conversion of glucose to MLA over various catalysts
been revealed that ultra-small WO
WO adsorbed onto Sn-Beta surface and resulted in the reduction
of silanol defects, which contributed to the increase of MLA yield.
3
particles leached from bulk
3
1
7
The catalytic performance of Mg-Sn-Beta was tested in the
conversion of glucose to MLA in methanol (Fig. 9). For comparison,
Mg-Beta, Sn-Beta and the binary catalyst of MgO and Sn-Beta was
also investigated. A negligible yield of MLA was obtained without
catalyst though 36% of glucose was converted. The main product is
fructose (17%) that derived from glucose isomerization. A small
amount of erythrose (5%) and glycolaldehyde (4%) derived from
retro-aldol of glucose are also detected. In addition, a few of other
We measured the leaching amount of Mg from MgO during
reaction by ICP method. Only 0.06 mol% of Mg was leached from
MgO, which implies MgO is much more stable than WO
3
(1.5 mol%
of W leaching from WO ). Probably this is the reason that bulk MgO
3
is an ineffective co-catalyst. Contrarily, the yield of MLA increases
dramatically over Mg-Sn-Beta zeolites. The yield of MLA increases
first and then decreases with the content of Mg. The highest MLA
yield (48%) is obtained over 1Mg-Sn-Beta. Turnover frequency (TOF)
−1
−1
value for MLA formation (molMLA mol L acid sites h ) from glucose
over Mg-Beta, Sn-Beta and Mg-Sn-Beta zeolites was measured and
given in Fig. 9b. The TOF value increases in the order of Mg-Beta <
Sn-Beta < 0.25Mg-Sn-Beta ≈ 4Mg-Sn-Beta < 1Mg-Sn-Beta. Mg-Sn-
Beta with less silanol defects gives higher activity for MLA formation
than Sn-Beta. Among Mg-Sn-Beta zeolites, 1Mg-Sn-Beta has the
least silanol defects and exhibits the highest activity for MLA
formation. The results suggest the amount of silanol defects plays a
crucial role in the conversion of glucose to MLA. The adverse effect
of the high-defect environment of Sn-Beta was also observed in the
1
00
(
a)
8
6
4
2
0
0
0
0
0
Conversion of glucose
Yield of methyl lactate
15
isomerization of glucose to fructose. Probably, the elimination of
silanol defects decreased the weak Brønsted acidity, reduced the
formation of byproducts and thus improved the selectivity of
k
O
M
Beta Beta Beta Beta Beta Beta Beta Beta
n
g
la
11
B
M
-
-
-
n
S
-
-
-
-
-
MLA.
g
n
-Sn
-Sn
-Sn
-Sn
-Sn
g
S
+
g
g
g
g
O
M
0.5M
M
M
M
g
0.25
1
2
4
M
Table 2 Isomerization of glucose in methanola
Catalyst Temp. Conv.
Yield (%)
100
o
(
b)
( C)
(%)
Fructos Mannos Other ML
e
e
0
0
0
5.1
6.8
6.9
7.8
0.7
3.8
4.9
5.3
C
0
0
0
0
1.4
2.6
2.9
1.4
2.1
4.6
8.5
3
A
0
0
0
1.1
1.6
1.8
2.6
1.6
3.2
7.3
19
8
6
4
2
0
0
0
0
0
Mg-Beta 90
00
10
80
6.0
7.0
10
46
70
91
97
19
45
60
80
2.2 (33)
4.4 (57)
6.3 (60)
28 (61)
43 (61)
53 (58)
56 (58)
11 (58)
18 (40)
20 (33)
20 (25)
1
1
Sn-Beta
9
0
100
10
1Mg-Sn- 80
1
eta
eta
Beta
Mg-
-B
ta
ta
n-B
e
-B
-Be
n
-S
Beta
90
100
110
n
Sn
g-
g-S
M
S
g
.
2
5
M
1M
4
0
Fig. 9 Conversion of glucose in methanol over various Beta zeolites.
Reaction conditions: glucose (0.37 g), catalyst (0.20 g), methanol
a
Reaction conditions: glucose (0.37 g), catalyst (0.20 g), methanol
15 mL), N (0.4 MPa), 1 h. The value in the parenthesis is the
o
o
(
1
15 mL), N
0% MLA yield. The binary catalyst is composed of Sn-Beta (0.20 g)
and MgO (0.10 g).
products (adducts of pyruvaldehyde with methanol, 2%) are also
2
(0.4 MPa), (a) 160 C for 5 h and (b) 140 C with about
(
2
selectivity of fructose.
The conversion of glucose to MLA is a reaction with multiple
steps including mainly (1) isomerization of glucose to fructose, (2)
retro-aldol of fructose to DHA and glyceraldehyde (GLA) and (3)
C
3
observed. Sn-Beta gives 99% of glucose conversion and 22% of MLA
yield. The low MLA yield over post-synthesized Sn-Beta is expected,
6
| J. Name., 2012, 00, 1-3
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isomerization of DHA and GLA to MLA (Scheme S1).10,17 The activity
of Mg-Sn-Beta for each step was studied and compared with Sn-
Beta and Mg-Beta. For isomerization of glucose in methanol (Table
View Article Online
DOI: 10.1039/C9CY02376C
100
(
a)
Mg-Beta
Sn-Beta
1Mg-Sn-Beta
8
0
2
), Mg-Beta alone shows low activity. Isomerization of glucose to
2
1,22,29,31
fructose is catalyzed by base or L acid.
Mg-Beta only
60
exhibits weak L acidity/basicity, so its activity is very low for
isomerization of glucose. Sn-Beta was much more active for glucose
isomerization to fructose. Glucose is converted quickly at moderate
temperature and fructose is a main product. The high activity of Sn-
Beta for glucose isomerization originates from the strong L acidity
of framework Sn sites which can form complex with glucose and
4
0
20
0
40
50
60
70
80
90
o
Reaction temperature ( C)
3
5
complete isomerization via 1,2-H-shift. In addition, a certain
amount of mannose, an epimer of glucose, was also detected. A
(
b)
8
6
0
0
small amount of C
3
products including MLA and other C
3
products
Mg-Beta
Sn-Beta
Mg-Sn-Beta
was also formed, which are the retro-aldol products of fructose.
Mg-Sn-Beta is less active and selective than Sn-Beta for glucose
isomerization to fructose. It has been proved that open Sn sites
1
40
3
6-
with adjacent silanols facilitate glucose isomerization to fructose.
2
0
0
3
8
The decrease of silanols in Mg-Sn-Beta explains the lower activity
and selectivity of Mg-Sn-Beta for isomerization of glucose to
fructose. However, the amount of C
over Mg-Sn-Beta.
3
products increases notably
40
50
60
70
80
90
o
Reaction temperature ( C)
The results of fructose conversion over Mg-Beta, Sn-Beta and Fig. 10 Isomerization of DHA to MLA over Mg-Beta, Sn-Beta and
Mg-Sn-Beta are presented in Table 3. The activity of Mg-Beta for 1Mg-Sn-Beta. Reaction conditions: DHA (0.37 g), catalyst (0.20 g),
1
the retro-aldol reaction of fructose is very low, and the yield of total methanol (15 mL), N (0.4 MPa), 1 h.
2
o
C
3
products was only 7% at 140 C for 1 h. Sn-Beta shows higher
activity for the retro-aldol reaction of fructose than Mg-Beta. 20%
yield of total C
larger than over Sn-Beta at the same temperature, indicating the
formation of MLA is faster over the former. The apparent activation
o
3
products is achieved at 140 C for 1 h. For 1Mg-Sn-
Beta, the selectivity for the retro-aldol products is much higher than
−1
energy over 1Mg-Sn-Beta and Sn-Beta is 73 and 119 kJ mol ,
respectively (Fig. S6). This means that introduction of magnesium
eliminated the proximal silanol of Sn site, lowered the activation
energy and facilitated the formation of MLA. The above results
prove that Mg-Sn-Beta promotes significantly the retro-aldol
reaction of fructose, which is the rate-determining step of glucose
Sn-Beta and 54% of yield of total C
3
compounds is obtained at 140
o
C for 1 h. The TOF values of Mg-Beta, Sn-Beta and 1Mg-Sn-Beta for
MLA formation from fructose were calculated based on the amount
of L acid sites (Table 3). The TOF values of the above catalysts
−1
−1
decrease in the order: 1Mg-Sn-Beta (208 h ) > Sn-Beta (40 h ) >
−1
Mg-Beta (11 h ). In addition, the kinetic studies of the retro-aldol
reaction of fructose over 1Mg-Sn-Beta were performed and
compared with Sn-Beta. The results are given in Fig. S4-6. Kinetic
results (Fig. S4 and Fig. S5) show that the formation of MLA from
the retro-aldol reaction of fructose fitted well with the pseudo-first-
order reaction kinetics, which is consistent with our previous
1
0
conversion to alkyl lactate. The possible mechanism of retro-aldol
of fructose over Mg-Sn-Beta is given in Scheme 2.
H
H
H
H
H
OH
O
Si
O
Sn O
H
OH
O
Si
O
Sn O
Si
O Si
Sn
O
Si
Si
O
Si
Si
O
H
OH
O
H
OH
O
O
HO
H
HO
H
O
H
O
O
Si
H
O
H Si
Si
H
H
Si
HO
H
H
HO
H
H
O
1
2,17
O
report.
The rate contant of MLA formation over 1Mg-Sn-Beta is
Mg
Mg
OH
OH
O
H
+
+
Mg
H
H
OH
H
O
H
H
O
CH2OH
CH2OH
H
OH
CH2OH
OH
Table 3 Retro-aldol of fructose in methanola
CH2OH
Scheme 2 Proposed mechanism of retro-aldol of fructose over Mg-
Sn-Beta.
The performance of Mg-Beta, Sn-Beta and Mg-Sn-Beta for
isomerization of DHA to MLA was also investigated (Fig. 10). The
activities of the three catalysts follow the order of Sn-Beta > Mg-Sn-
TOF
(h )
Yield (%)
Other
−
1 b
Temp.
Conv.
(%)
Catalyst
o
Total
C
3
(
C)
MLA
C
3
1
1
1
1
120
140
20
40
20
40
25
39
48
55
80
90
1.7
3.7
1.7
11
26
37
0
1.7
7.1
3.0
20
38
54
Beta > Mg-Beta. The conversion of DHA to MLA is also a reaction
Mg-Beta
Sn-Beta
11
40
3.4
1.3
8.6
12
32,39
catalyzed by L acid.
But the L acidity of Mg-Beta is too low to
catalyze the conversion of DHA to MLA. The L acidity of Mg-Sn-Beta
is higher than Sn-Beta; but Mg-Sn-Beta has less silanols, which
1
Mg-Sn-
32
restrained its activity for the conversion of DHA to MLA.
2
08
Beta
17
Based on the above results, it can be concluded that compared to
Sn-Beta, Mg-Sn-Beta has a significant promotion effect on the retro-
aldol of fructose, which is the rate-determining step of glucose
conversion to MLA, therefore the yield of MLA is improved.
a
Reaction conditions: fructose (0.37 g), catalyst (0.20 g), methanol
b
(15 mL), N
2
(0.4 MPa), 1 h. Defined as mole MLA generated per
o
mole L acid sites per hour. The reaction was performed at 140 C
and the yield of MLA was about 10%.
3
.2.2 Effect of reaction conditions
Subsequently, the reaction temperature and time were optimized
(
Fig. 11). The yield of MLA increases with the reaction temperature
o
until 170 C. After that the yield of MLA decreases slightly due to
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the formation of byproducts such as humins. The yield of MLA
increases with the reaction time, and reaches 50% at 170 C for 5 h.
Further prolonging the reaction time the yield of MLA almost
unchanged. The yield (50%) of MLA from glucose over 1Mg-Sn-Beta
View Article Online
DOI: 10.1039/C9CY02376C
o
100
80
Conversion
MLA
Other C3
Fructose
Mannose
Erythrose
LA
1
0
was higher than that over Sn-Beta synthesized in fluoride (43%),
2
7
which is also higher than over nanosized Sn-Beta (41%). Moreover,
the preparation of Mg-Sn-Beta by coimpregnation method is more
60
40
20
0
1
1
simple. In comparison with alkali ion modified Sn-Beta catalyst
Glycolaldehyde
1
7
3
and the binary catalyst of WO and Sn-Beta, Mg species are not
6
5
4
3
2
1
0
0
0
0
0
0
0
(
a)
50
100
150
200
250
300
Reaction time (min)
Fig. 12 Conversion of glucose in methanol over Mg-Sn-Beta.
Reaction conditions: glucose (0.37 g), 1Mg-Sn-Beta (0.20 g),
o
methanol (15 mL), N
2
(0.4 MPa), 120 C.
1
20
140
160
170
180
70
60
50
40
30
20
10
0
o
Temperature ( C)
6
5
4
3
2
1
0
0
0
0
0
0
0
(
b)
1
2
3
4
5
Reaction runs
0
2
4
6
8
10
Reaction time (h)
Fig. 13 Reusability of 1Mg-Sn-Beta catalyst.
Reaction conditions: glucose (0.37 g), 1Mg-Sn-Beta (0.20 g),
6
5
4
3
2
1
0
0
0
0
0
0
0
o
(
c)
methanol (15 mL), N (0.4 MPa), 120 C, 0.5 h.
2
when the concentration of glucose is 12.3 wt.% (1.48 g of glucose in
1
3
5 mL of methanol), which is higher than over WO combined Sn-
1
7
Beta binary catalyst.
3
.2.3 Product evolution with reaction time
0
.37
0.74
1.11
1.20
1.48
Amount of glucose (g)
o
A time-course study was further conducted at 120 C to examine
the products evolution over Mg-Sn-Beta (Fig. 12). Glucose
isomerizes fast to fructose and at the same time a few of mannose,
erythrose and glycolaldehyde is also formed. With the time lapse,
Fig. 11 Effects of reaction temperature (a), time (b) and glucose
concentration (c) on the yield of MLA. Reaction conditions: glucose
(
5
2
0.37 g), 1Mg-Sn-Beta (0.20 g), methanol (15 mL), N (0.4 MPa), (a)
the formed fructose is further converted to the C
including MLA, other C and a few of LA. The amount of mannose,
erythrose, glycolaldehyde and LA is always in low level (~5%) in the
3
products
o
o
h, (b) 170 C and (c) 160 C for 5 h.
3
easy to leach than alkali ion and W species, so Mg-Sn-Beta will be investigated period. The amount of MLA and other C products
more stable. Mg-Sn-Beta can also transform glucose solution in high keeps similar change trend. It is found that increasing temperature
3
concentration. As shown in Fig. 11c, 40% of MLA yield is obtained
promotes the formation of MLA (Fig. 11a), whereas almost does not
affect other C products (Fig. S7). This suggests the formation of
MLA and other C products are two competitive reactions and the
conversion of other C products to MLA via back reaction is
3
3
3
4
0
difficult.
3
.2.4 Reusability of Mg-Sn-Beta zeolite
Finally, the reusability of 1Mg-Sn-Beta was investigated, which was
evaluated according to the initial activity of MLA formation. After
each run the catalyst was recovered by centrifugation, washed with
methanol for three times, dried at ambient temperature, calcined
8
| J. Name., 2012, 00, 1-3
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o
at 550 C for 1 h and then used for the next run. As shown in Fig. 13
Osmundsen, R. J. Davis, E. Taarning, P. Fristrup anVdiewMA.rtSic.leHOonllmine,
J. Mater. Chem. A, 2014, 2, 20252-20262.
and Fig. S8, the TOF value of MLA formation, the conversion of
glucose and the yield of MLA only decreased slightly after five runs.
The catalyst recovered from the fifth reaction was washed with
methanol, dried and calcined at 550 C for 2 h, and then it was
2
characterized by XRD, N physisorption and ICP (Fig. S9 and Table
S1). The results show that the crystallinity, surface area and pore
volume of reused 1Mg-Sn-Beta decreased slightly after five runs.
ICP results show that 2.6% of Sn and 2.0% of Mg was leached from
Mg-Sn-Beta, respectively, after five reaction runs, suggesting Sn 18 X. Lu, L. Wang and X. Lu, Catal. Commun., 2018, 110, 23-27.
and Mg are not facile to leach from the catalyst. These results 19 C. P. Cabello, G. Gûmez-Pozuelo, M. Opanasenko, P. Nachtigall
DOI: 10.1039/C9CY02376C
1
1
1
1
4 P. Y. Dapsens, C. Mondelli and J. Pérez-Ramírez, Chem. Soc.
Rev., 2015, 44, 7025-7043.
5 J. C. Vega-Vila, J. W. Harris and R. Gounder, J. Catal., 2016, 344,
o
1
08–120.
6 W. Dong, Z. Shen, B. Peng, M. Gu, X. Zhou, B. Xiang and Y.
Zhang, Sci. Rep., 2016, 6, 26713.
7 X. Yang, Y. Zhang, L. Zhou, B. Gao, T. Lu, Y. Su and J. Xu, Food
Chem., 2019, 289, 285-291.
1
indicate that the catalyst is stable and recyclable.
and J. Čejka, ChemPlusChem, 2016, 81, 828-835.
2
2
0 Y. Zhou, Y. Jin, M. Wang, W. Zhang, J. Xie, J. Gu, H. Wen, J. Wang
and L. Peng, Chem. Eur. J., 2015, 21, 15412-15420.
Conclusions
1 I. Delidovich and R. Palkovits, Catal. Sci. Technol., 2014, 4, 4322-
4
329.
Mg-Sn-Beta zeolite with different Mg/Sn molar ratio prepared from 22 I. Graça, D. Iruretagoyena and D. Chadwick, Appl. Catal. B, 2017,
2
06, 434-443.
3 J. Shin, D. Jo and S. B. Hong, Accounts Chem. Res., 2019, 52,
419-1427.
deAl-Beta by coimpregnation method has higher L acidity/basicity
and less silanol defects than Sn-Beta. Both Mg and Sn can be
incorporated into the framework sites of Beta zeolite and generate
L acid sites, but the L acidity of framework Mg is weaker than
framework Sn . The increase in L basicity results from O
connected to Mg . For glucose conversion to MLA, Mg-Sn-Beta
zeolite promoted the rate-determining step of retro-aldol of
fructose, and thus improved the selectivity of MLA.
2
2
2
2
2
1
4 R. Xu, W. Pang, Q. Huo, Chemistry of Zeolites and Related
Porous Materials, Science Press, Beijing, 2nd ed., 2014, p. 164.
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Catalysis Science & Technology
Page 10 of 10
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DOI: 10.1039/C9CY02376C
Mg-Sn-Beta zeolite with different Mg/Sn molar ratio was prepared from the parent
deAl-Beta by coimpregnation method. It shows higher selectivity for conversion of
glucose to methyl lactate than post-synthesized Sn-Beta.
OH
HO
OH
OH
O
O
O
O Si O Si O Si O Mg
O
O
O
O
O
O Si O Si O Si O Sn O
Glucose
O
O
O
O
o
1
70 C, 5 h
OH
O Si O Mg O Si O Si O
CH OH
3
O
O
O
O
O Si O Si O Sn O Si O
OH
O
O
O
O
O
Yield 50%
OCH3
Mg-Sn-Beta
Methyl lactate